Dibasic organic acid producing strain and preparation and application of same

ABSTRACT

Provided are an engineered strain for synthesizing a dibasic organic acid and preparation and application of same. The engineered strain introduces or up-regulates expression of a positive regulator gene for synthesis of a dibasic organic acid, and/or down-regulates expression of a negative regulator gene for synthesis of a dibasic organic acid, as compared with the origin strain of the engineered strain, the producing capability for producing the dibasic organic acid is improved. The dibasic organic acid comprises malic acid, succinic acid, fumaric acid, oxaloacetic acid, glutaric acid, and adipic acid; the expression product of the positive regulator gene comprises aspartate aminotransferase, glutamic acid-aspartate transporter, C4-dicarboxylic acid transporter, pyruvate carboxylase and malate dehydrogenase, glucose transporter; the expression product of the negative regulatory gene comprises succinyl-CoA synthase, and malic acid-alpha ketoglutarate transporter, and the original strain comprises  Myceliophthora thermophila, Thielavia terrestris, Aspergillus , and  Rhizopus.

TECHNICAL FIELD

The present invention relates to the field of biotechnology and bioengineering. In particular, the present invention relates to a new engineered bacterium producing dibasic organic acid, and a method for preparing dibasic organic acid by using the same.

BACKGROUND

In view of the rapid growth in the demand for petroleum-based chemicals or fuels and their increasing costs, and considering the impact of geopolitical instability on crude oil prices and the impact of greenhouse gas emissions on global climate, there is an urgent need to develop a new green process with renewable and sustainable properties to produce these petroleum-based chemicals or fuels. These factors have greatly contributed to the research of using enormous biomass to produce chemicals or fuels, especially using non-food renewable resources as raw materials (second-generation biorefinery).

At present, the common process for biomass utilization is divided into three steps: biomass pretreatment, enzymatic hydrolysis and fermentation, wherein the pretreatment still requires high energy consumption and high pollution processes such as high temperature, high pressure, or acid and alkali treatment. In addition, although the yield of cellulolytic enzymes production has reached above the level of 100 g/L, but the application of cellulase in enzymolysis costs too much, and takes a large proportion in the whole process costs, which does not meet the basic requirements of industrial production in large scale. In the practical application, the production process of biomass-based products is greener, more sustainable, and more in line with the trend of modern industrial development, but its production costs are much higher than that of petroleum-based products. The development of bio-refining industry is seriously restricted with the economic factors.

DL-malic acid, the product of malic acid as an example of organic dicarboxylic acid, is triditionally completed by chemical catalytic synthesis based on petroleum-based materials, and it's application is limited in the medicine and food industry because L-malic acid needs to be obtained through optical resolution. The production of single optically active L-malic acid by microbial fermentation draws great concerns and attention.

At present, there are still a lot of problems for malic acid fermentation, for example, the temperature for malic acid fermentation is low, therefore the fermentation reaction needs to be continued after cooling down due to the excessive heat produced during the conventional fermentation process. This not only restricted the fermentation efficiency, but also wasted energy. For another example, the cost is higher due to the substrate of glucose, while malic acid with a production level can not be obtained by cheap substrate.

Therefore, there is an urgent need to develop a method for producing an organic binary acid, especially malic acid, effectively using cheap substrates.

SUMMARY OF THE INVENTION

The invention provides a new engineered strain for synthesizing a dibasic organic acid in high yield, and a preparation method and an application thereof.

In the first aspect, the invention provides an engineered strain with genetic modification for synthesizing a dibasic organic acid, which introduces or up-regulates the expression of a positive regulator gene for a dibasic organic acid synthesis (preferrably introduces an exogenous positive regulator gene), and/or down-regulates expression of a negative regulator gene of a dibasic organic acid synthesis, and as compared with the origin strain of the engineered strain, the capability for producing the dibasic organic acid is significantly improved, wherein, the dibasic organic acids comprise malic acid, succinic acid, fumaric acid, oxaloacetic acid, glutaric acid, or adipic acid.

In another preferred example, the dibasic organic acid is malic acid.

In another preferred embodiment, the dibasic organic acid is a C4-C6 dibasic acid.

In another preferred example, the producing capacity of the dibasic organic acid is industrial grade.

In another preferred embodiment, the original strain of the engineered strains comprises Myceliophthora strains, Thielavia, Aspergillus or Rhizopus; preferably, Myceliophthora comprises Myceliophthora thermophila or Myceliophthora heterothallica; and Myceliophthora thermophila is preferred; Thielavia comprises Thielavia terrestris; Aspergillus comprises Aspergillus oryzae, Aspergillus flavus, Aspergillus sojae; Rhizopus comprises Rhizopus oryzae Went et Pr.Geerl.

In another preferred embodiment, among the original strain genomes, each corresponding positive and/or negative regulator gene of dibasic organic acid synthesis has at least 92%, preferably at least 95%, preferably at least 98%, 99% of homology.

In another preferred embodiment, the term “significantly improve” means that compared to the original strain, the engineered strain provides a yield of dibasic organic acid fermentation at least more than 10 g/L, preferably at least 10-50 g/L; more preferably at least 50-300 g/L of the based on the volume of the fermentation liquid; and/or

the term “significantly improve” means that compared to the original strain, the engineered strain increases or improves the dibasic organic acid producing capacity by a at least 10%; preferably at least 10-50%; more preferably at least 50%-500%. In another preferred embodiment, the expression product of the positive regulator gene comprises one or more polypeptides or the derivative polypeptides thereof selected from the group consisting of aspartate aminotransferase, glutamate-aspartate transporter, and glucose transporter; and/or

the expression product of the negative regulator gene comprises one or more polypeptides or derived polypeptides thereof selected from the group consisting of Succinyl-CoA synthase, and Malic acid-alpha ketoglutarate transporter.

In another preferred embodiment, the aspartate aminotransferase is as set forth by SEQ ID NO.: 4.

In another preferred embodiment, the glutamate aspartate transporter is as set forth by SEQ ID NO.: 6.

In another preferred example, the malate dehydrogenase is as set forth by SEQ ID NO.: 10.

In another preferred example, the glucose transporter is as set forth by SEQ ID NO.: 96.

In another preferred example, the succinyl-CoA synthase is as set forth by SEQ ID NO.: 2.

In another preferred example, the malic acid-alpha ketoglutarate transporter is as set forth by SEQ ID NO.: 8.

In another preferred embodiment, an exogenous positive regulator gene for synthesizing the dibasic organic acid is introduced into the engineered strain, and the negative regulator gene of which for synthesizing the dibasic organic acid is simultaneously down regulated.

In another preferred embodiment, the expression product of the positive regulator gene also comprises one or more polypeptides or their derivative peptides selected from the group consisting of C4-dicarboxylic acid transporter, pyruvate carboxylase, malate dehydrogenase, glucose transporter and the combinations thereof.

In another preferred embodiment, the engineering strains are obtained by the following methods:

introducing a positive regulatory gene for synthesizing the dibasic organic acid into an original strain (preferably introducing an exogenous positive regulatory gene) or up-regulating the expression of a positive regulatory gene for synthesizing the dibasic organic acid in an original strain; or down-regulating the expression of a negitive regulatory gene for synthesizing the dibasic organic acid in an original strain.

In another preferred embodiment, the polypeptides or the derived polypeptides thereof are selected from the group consisting of

(I) one or more sequences of SEQ ID NO.: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 26, or 96;

(II) one or more polypeptides derived from (1) by one or several amino acids deleted, added or substituted from the sequence of SEQ ID NO.: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 26 or 96, and enabling the engineered strain to have a capacity for producing dibasic organic acid; and

(III) one or more polypeptides that have an amino acid sequence having an identity of ≥90% (preferably ≥95%, more preferably ≥98%) with the sequence of SEQ ID NO.: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 26, or 96 and enabling the engineered strain to have a capacity for producing dibasic organic acid.

In another preferred embodiment, a polynucleotide sequence encoding the polypeptide or its derived polypeptide comprises:

(i) a polynucleotide encoding a sequence of SEQ ID NO.: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 26, 28, 30, or 96;

(ii) a polynucleotide having a sequence of SEQ ID NO.: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 25, or 95;

(iii) a polynucleotide sequence having an identity of ≥95% (preferably ≥98%) with a sequence of SEQ ID NO.: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 25, or 95; or

(iv) a polynucleotide derived from SEQ ID NO.: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 25, or 95 by 1-60 (preferably 1-30, more preferably 1-10) nucleotides deleted or added at 5′ end and/or 3′ end;

(v) a polynucleotide complementary to any one of the sequence of (I)-(IV).

In another preferred embodiment, in the engineered strain with up-regulated expression or introduction of an exogenous positive regulatory gene for synthesizing the dibasic organic acid, compared to the original strain (wild type), the expression of the positive regulator gene is improved by at least 50%, preferably at least 60%, 70%, 80%, 90%, or 100%.

In another preferred embodiment, in the engineered strain with down-regulated expression of a negative regulatory gene for synthesizing the dibasic organic acid, compared to the original strain (wild type), the expression of the negative regulator gene is decreased by at least 50%, preferably at least 60%, 70%, 80%, 90%, or 100%.

In the second aspect of the invention, a method for preparing a dibasic organic acid is provided, comprising steps of:

(i) providing the engineered strain described in the first aspect of the invention;

(ii) in the presence of a substrate, culturing the engineered strain described in (i), thereby obtaining a fermentation product containing a dibasic organic acid; and optionally

(iii) isolating and purifying the fermentation product obtained in (ii) thereby further obtaining the dibasic organic acid.

In another preferred embodiment, the substrate comprises monosaccharide(s), polysaccharide(s), glycan(s), biomass, or combinations thereof.

In another preferred embodiment, the polysaccharide comprises sucrose, maltose, cellobiose, fibrous oligosaccharides, xylobiose, xylosaccharides or combinations thereof.

In another preferred embodiment, the monosaccharide comprises glucose, xylose, arabinose, or combinations thereof.

In another preferred embodiment, the glycan comprises cellulose, crystalline cellulose, hemicellulose, starch or combinations thereof.

In another preferred embodiment, the culture temperature of the engineered strain is 25-60° C., preferably 40-55° C., more preferably 45-50° C.

In the third aspect of the invention, a method is provided for preparing the engineered strain of the first aspect of the invention, and/or giving a capacity to a Myceliophthora strain or enhancing the capacity of a Myceliophthora strain for producing a dibasic organic acid, comprising steps of:

introducing or up-regulating the expression of a positive regulatory gene for synthesizing the dibasic organic acid (preferably introducing an exogenous positive regulatory gene) in an original strain; and/or down-regulating the expression of a negitive regulatory gene for synthesizing the dibasic organic acid in an original strain, thereby preparing an engineered strain of the first aspect of the invention and/or allowing Myceliophthora strain to synthesize the dibasic organic acid.

In another preferred embodiment, the method comprises steps of:

(a1) providing an expression vector carrying an exogenous positive regulatory gene for synthesizing the dibasic organic acid;

(b1) transferring the expression vector into host cells;

(c1) culturing the host cells; and/or the method comprises steps of:

(a2) knocking out the negative regulatory gene for synthesizing the dibasic organic acid in host cells;

(b2) culturing the host cells.

In the fourth aspect of the invention, combinations of expression products of a dibasic organic acid producing regulatory gene are provided, comprising at least two polypeptides selected from the group consisting of:

(Ia) a sequence of SEQ ID NO.: 4, 6, 10 or combinations thereof;

(IIa) polypeptides which are derived from (Ia) by one or more amino acids deleted, added or substituted from the sequence of SEQ ID NO.: 4, 6, or 10 and enable Myceliophthora strains to have a capacity and/or enhance the capacity for producing dibasic organic acid; and

(Ib) the sequences of SEQ ID NO.: 12, 14, 16, 18, 20, 22, 26, 28, 30, or 96 or combinations thereof; and

(IIb) polypeptides which are derived from (Ib) by one or more amino acids deleted, added or substituted from the sequence of SEQ ID NO.: 12, 14, 16, 18, 20, 22, 26, 28, 30, or 96 and enable Myceliophthora strains to have a capacity for producing dibasic organic acid and/or enhance the capacity for producing dibasic organic acid; and

In another preferred embodiment, the combination comprises at least the sequences of SEQ ID NO.: 4 and 6.

In another preferred embodiment, the combination comprises at least the sequences of SEQ ID NO.: 6 and 10.

In another preferred embodiment, the combination comprises at least the sequences of SEQ ID NO.: 4 and 10.

The fifth aspect of the invention provides a combination of the dibasic organic acid producing regulatory genes, which comprises at least two kinds of polynucleotides that encode the expression product in the combination of expression products of the fourth aspect of the invention.

In the sixth aspect of the invention provides a carrier comprising the combination of the fifth aspect of the invention, and/or containing an inhibitor that inhibits the negative regulator gene of the dibasic organic acid production.

In another preferred embodiment, the inhibitor is an interference RNA or antisense nucleic acid of the negative regulator gene (such as Succinyl-CoA synthase) of a dibasic organic acid production.

In another preferred embodiment, the sequence of the interfering RNA is as set forth by SEQ ID NO.: 74 or 75.

In another preferred embodiment, there are one or more carriers.

In the seventh aspect of the invention, a host cell is provided that has characteristics selected from the group consisting of:

(a1) containing the carrier of the first aspect of the invention;

(b1) the chromosomes of the host cells are artificially integrated with the polynucleotide that encodes the polypeptide of SEQ ID NO.: 4, 6, 10, or 96 or the expression of the original gene encoding said polypeptide is up-regulated; or the gene encoding the polypeptide of SEQ ID NO.: 2 and/or 8 in the chromosome of the host cell is knocked out or weakened; and optionally

the chromosomes of the host cells are integrated with one or more polynucleotide(s) selected from the polypeptides of SEQ ID NO.: 4, 6, 10, 12, 14, 16, 18, 20, 22, 26, and 96.

In another preferred embodiment, the host cell is the engineered strain of the first aspect of the invention.

In another preferred embodiment, the host cell is Myceliophthora strain, preferably Myceliophthora thermophila.

The eighth aspect of the invention, a use of the combination of the forth aspect of the invention is provided for preparing the engineered strain of the first aspect of the invention, and/or giving a capacity to Myceliophthora strain or enhancing the capacity of Myceliophthora strain for producing dibasic organic acid.

In another preferred embodiment, said “giving” or “enhancing” the capacity for producing dibasic organic acid refers to that after reconstruction the strain originally having no capability for producing and/or accumulating dibasic organic acid ability has a capacity for industrially producing dibasic organic acid, and/or the strain originally having poor capability for producing and/or accumulating dibasic organic acid has an enhanced capacity for industrially producing dibasic organic acid.

In the ninth aspect of the invention, an engineered strain with genetic modification for synthesizing the dibasic organic acid is provided, said engineered strain allows the dibasic organic acid to be obtained in a fermentation temperature of 25-60° C. by using glycan and/or biomass as a fermentation substrate,

wherein the original strain of the engineered strain is Myceliophthora;

and the dibasic organic acid comprises malic acid, succinic acid or fumaric acid.

In another preferred embodiment, the dibasic acid also comprises oxaloacetic acid, glutaric acid, or adipic acid.

In another preferred embodiment, the substrate comprises monosaccharides, polysaccharides, or the combinations thereof.

In another preferred embodiment, the engineered strain is artificially integrated with the positive regulatory gene for synthesizing the dibasic organic acid or up-regulates the expression of the positive regulatory gene for synthesizing the dibasic organic acid, and/or expresses down-regulatedly the negative regulatory gene of the dibasic organic acid synthesis, and compared with the original strain, the capacity of the engineered strain for producing dibasic organic acid is significantly improved.

In another preferred embodiment, the glycan comprises cellulose, crystalline cellulose, hemicellulose, starch (preferably corn, cassava, wheat) or combinations thereof.

The biomass comprises crop straw, forestry waste, papermaking industry waste, cotton textile industry waste, energy plant or part or all of its decomposition products; wherein the crop straw comprises corn straw, wheat straw, rice straw, sorghum straw, soybean straw, cotton straw, bagasse, or corncob; the forestry waste comprised branches, leaves, or sawdust; the papermaking industry waste comprises pulp slag, pulp waste; the cotton textile industry waste comprises wasten cotton and cotton textiles; the energy plants comprises sweet sorghum, switchgrass, miscanthus, reed or combinations thereof.

In another preferred embodiment, the substrate only comprises glycans and/or biomass.

In another preferred embodiment, the fermentation temperature is 40-55° C., preferably 45-53° C., more preferably 48-50° C.

In another preferred embodiment, the dibasic organic acid is malic acid.

In another preferred embodiment, the dibasic organic acid is a C4-C6 dibasic acid.

In another preferred embodiment, the producing capacity of the dibasic organic acid is industrial grade.

In the tenth aspect of the invention, a method for preparing a dibasic organic acid is provided, comprising steps of:

(i) providing the engineered strain of the ninth aspect of the invention;

(ii) in the presence of a substrate, culturing the engineered strain of (i), thereby obtaining a fermentation product containing dibasic organic acid, wherein the fermentation temperature is 25-60° C.; and optionally

(iii) isolating and purifying the fermentation product obtained in (ii) thereby further obtaining the dibasic organic acid;

wherein, the substrate comprises glycans and/or biomass.

In another preferred embodiment, the culture temperature of the engineered strain is 40-55° C., preferably 45-52° C., more preferably 48-50° C.

In another preferred embodiment, the substrate is cellulose, hemicellulose, starch, or biomass.

In another preferred embodiment, the substrate further comprises monosaccharides, polysaccharides, or combinations thereof.

In another preferred embodiment, the polysaccharide comprises sucrose, maltose, cellobiose, fibrous oligosaccharides, xylobiose, xylosaccharides or combinations thereof.

In another preferred embodiment, the monosaccharide comprises glucose, xylose, arabinose, or combinations thereof.

In the present invention, the producing capacity comprises, but is not limited to, the fermentation product concentration (titer), and/or conversion (yield), and/or the fermentation yield (productivity).

It should be understand that within the scope of the invention each of the technical features described in detail above and below (such as the examples) can be combined with each other separately, so as to form a new or preferred technical proposal, which will no longer be described herein due to the length limitation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a physical map of the mae gene expression vector pAN52-mae.

FIG. 2 shows a physical map of the expressing vector pAN52-TB-Ptef.

FIG. 3 shows a physical map of the mae gene and pyc gene co-expression vector pAN52-mae-pyc.

FIG. 4 shows a physical map of the mdh gene expression vector pAN52-mdh.

FIG. 5 shows a physical map of a binary carrier pAN52-SCLsilent-A.

FIG. 6 shows a physical map of a binary carrier pAN52-SCLsilent-B.

FIG. 7 shows a physical map of the knocking out carrier pPK2sur-barGFP::odc.

FIG. 8 is a physical map of the plasmid pMF272.

FIG. 9 shows the malic acid yield map in different strains on the eighth day with crystalline cellulose as a carbon source.

FIG. 10 shows a malic acid yield map in M. thermophila which overexpresses C4-dicarboxylic acid transporter.

DETAILED DESCRIPTION OF THE INVENTION

After extensive and thorough research, the inventors firstly and surprisingly discover a set of regulatory genes for producing or synthesizing a dibasic organic acid in filamentous fungi strains, especially Myceliophthora strains (such as Myceliophthora thermophilic), wherein, genes having a function of positive regulation comprise aspartate aminotransferase, glutamic acid-aspartate transporter, malate dehydrogenase, C4-dicarboxylic acid transporter, pyruvate carboxylase, glucose transporter, or combinations thereof, genes having a function of negative regulation comprise succinyl-CoA synthase, malic acid-alpha ketoglutarate transporter, or combinations thereof. It has been confirmed by the inventors that by up-regulation of one or more of the positive regulator genes and\or down-regulation of one or more of the negative regulator genes, the genetic modified engineered strain can effectively use monosaccharide, polysaccharide, glycans or mixed sugars, especially cheap polysaccharide (such as cellulose, etc.) to synthesize dibasic organic acid in high yield under high temperature conditions. In addition, the inventors also have confirmed by experiments that the regulatory function has relative strain species specificity. On this basis, the invention is completed.

Dibasic Organic Acid

As used herein, the term “dibasic organic acid” refers to organic acids in which each molecule can ionize two and only two hydrogen ions in water. The dibasic organic acid which can be used herein comprises C4-C6 dibasic organic acid, preferably C4-C5 dibasic organic acid, such as malic acid, succinic acid, fumaric acid, oxaloacetic acid, glutaric acid, or adipic acid. Preferably, the dibasic organic acid of the invention comprises malic acid or succinic acid.

Taking Malic acid for an example, L-malic acid is an important natural organic acid, and widely used in food, beverages, spices, healthcare, chemicals, plastics and other industries. In the food industry, L-malic acid can be used as an acidity regulator, a food preservative, a food deodoriant, or a pasta enhancer, and in the pharmaceutical industry, L-malic acid can be added in drug injections, preparations, tablets, or syrups for helping to improve the utilization rate of the medicine. In the daily chemical industry, it can be used as an ingredient of deodorants and detergents. Malic acid has an important position and plays an important role in the organic acid industry. In recent years, the demand for malic acid in the international market increases rapidly, and the market prospect is bright.

Traditionally, the production of malic acid is completed by chemical catalytic synthesis based on petroleum-based materials, and the product is DL-malic acid which limits its application in medicine and food industry because L-malic acid needs to be obtained through optical resolution. The production of L-malic acid with single optical rotation by microbial fermentation has been widely concerned and highly valued.

In addition, the inventors have found that not only the malic acid (and even organic acid) fermentation in Myceliophthora can be enhanced through regulating multiple new genes, but also the organic acid producing capacity of strains (including Aspergillus (preferably Aspergillus oryzae, Aspergillus sojae, Aspergillus terreus, Aspergillus niger), Rhizopus (preferably Rhizopus oryzae), beside the Myceliophthora with accumulation ability) can be improved by genetic modification.

“a capacity for producing organic acid” herein refers to the industrialized capacity for producing organic acid, which is equivalent to the term of “industrial production level”, “industrialized potential”, “industrial producing capacity”, or “organic acid producing capacity”, which can be used interchangeably, and refers to the total volume of fermentation liquid, the fermentation yield is at least 10 g/L, preferably at least 15-40 g/L; more preferably, at least 50-300 g/L, and any integer and non-integer in this range, which is no longer listed one by one.

For the fermentation of malic acid and other organic acids, the traditional dominant strain is aspergillus strain. In addition, some traditional dominant strains of organic acids include but not limited to: Citric acid-Aspergillus niger, Malic acid-Aspergillus flavus, Aspergillus oryzae, or Lactic acid-Rhizopus oryzae. But Myceliophthora does not belong to common strains which accumulating organic acids. The invention has shown that for strains (such as Neurospora crassa, Trichoderma reesei) that have few accumulation of organic acids (usually no more than the grade of gram/liter) in natural conditions, the modification of the synthesizing rout of organic acids (such as malic acid) will not effectively improve their producing yield to industrial level (10 g/l or more), but in the case of strains that did not accumulate organic acids, i.e Myceliophthora strains (Myceliophthora thermophila, Myceliophthora heterothallica), the capacity for synthesizing organic acid (malic acid) was significantly improved (10 g/I or more) by genetic modification, which is very surprising.

Substrate

As used herein, the term “substrate” refers to carbohydrates that can produce a dibasic organic acid in the presence of filamentous fungi, including monosaccharides, polysaccharides, glycans, biomass or a combination thereof, wherein the term “monosaccharide” includes but not limited to glucose, xylose, Arabia sugar or a combination thereof; “polysaccharide” includes but not limited to sucrose, cellobiose, cello-oligosaccharides, xylobiose, xylo-oligosaccharides or a combination thereof, wherein the “glycans” includes but not limited to cellulose (especially cellulose from biomass source), hemicellulose, or its combination; biomass includes but not limited to crop straw, forestry waste, paper-making industry waste, energy plant, or a combination thereof. Examples of preferred substrates are described below:

Glucose, xylose, and Arabia sugar are three important monosaccharides. Glucose (chemical formula is C₆H₁₂O₆) is also known as corn glucose, corn sugar, referred to as glucose, is one of the most widely distributed and most important monosaccharides in nature. Glucose plays an important role in biology field. It is the energy source and metabolic intermediate product of living cells, that is, the main energy supply of biological substances. It has been widely used in confectionery manufacturing and medicine field. It can be largely prepared in industry by using corn, cassava, etc. as raw materials.

Xylose is a five carbon pentose and is the main monosaccharide that makes up hemicellulose, therefore, xylose also widely exists in abandoned parts of agricultural products such as corn cob, straw, the skin of cotton boll and others. It can be obtained by hydrolysis from hemicellulose in biomass.

Arabia sugar, also called pectose, often exists in the form of heteropolysaccharide in combined with other monosaccharides. Arabia sugar exists in the cereale such as cornmeal, corncob, rice, wheat etc. and in hemicelluloses and pectic substances in the cell walls of the plants such as beet, apple and others. Xylose and arabinose are the most important five carbon sugars obtained after the degradation or pretreatment of biomass. Microorganisms are usually difficult to be used, which is the difficulty in utilizing whole sugar of biomass.

Sucrose, cellobiose and xylobiose are three important disaccharides. Sucrose is the main product of photosynthesis, and widely distributed in plants, especially in sugar beet, sugar cane and fruit, in which the content is extremely high. Sucrose is a disaccharide which is formed by dehydration condensation of one molecule of glucose and one molecule of fructose and widely used in the biological fermentation industry. It is the raw material of various products such as alcohol, citric acid, lactic acid, glycerol, alcohol, medicine and others. Whereas the cellulose is composed of cellobiose, which can be degraded from cellulose further hydrolyzed into two molecules of glucose. Xylobiose is a xylo-oligosaccharide formed by two xylose through beta-1,4-glycosidic bond, and is a linear disaccharide. It can be obtained by hydrolysis of hemicellulose and can further be decomposed into two xyloses.

Cello-oligosaccharides and xylosaccharides are two important oligosaccharides. Cello-oligosaccharides usually refer to oligosaccharides produced by glucose through the linkage of beta-1,4 glycosidic bonds. Xylosaccharides, also known as xylo oligosaccharides, are oligosaccharides formed by 2-7 of D-xylose through the linkage of beta-1,4-glycosidic bonds, and some may also contain arabinose, glucuronic acid and other side chains. Xylobioses, xylosaccharides, cello-oligosaccharides and cellobioses are the main products of cellulose and hemicellulose in plant cellulose (corn cob, bagasse, straw, etc.) through hydrolysis.

Biomass mainly contains cellulose, hemicellulose and lignin. All kinds of crop and energy plant straws (corn straw, wheat straw, rice straw, sorghum straw, bagasse, miscanthus etc.), forestry wastes (sawdust, branches and leaves), papermaking industry waste and so on are important biomass resources. Under certain conditions, they can be degraded into glycans (such as xylan, glucan), oligosaccharides and monosaccharides which can be used by some microbial fermentation. Developing the use of available biomass hydrolysates or even the biomass after simple pretreatment as a carbon source to produce chemical products (ethanol, organic acids, etc.) through fermentation is an important research at home and abroad.

The Dibasic Organic Acid Synthesis Regulatory Gene and its Expression Product

As used herein, the term “the dibasic organic acid synthesis regulatory gene” and “the polynucleotides that encode the polypeptides of the invention” can be used interchangeably, and includes “the positive regulatory gene and the negative regulator gene for synthesizing the dibasic organic acid”. Wherein the terms “positive regulatory gene for synthesizing the dibasic organic acid”, “positive regulatory gene” and “overexpressing gene” can be used interchangeably and refer to one or more positive genes capable of promoting or enhancing the synthesis of the dibasic organic acids in filamentous fungi (e.g., Myceliophthora, Aspergillus oryzae, Aspergillus sojae, Aspergillus terreus, etc.); the terms “negative regulatory gene for synthesizing the dibasic organic acid” and “the negative regulatory gene” refer to one or more negative genes capable of inhibiting or reducing the synthesis of the dibasic organic acids in filamentous fungi; the terms “introduction” and “artificial integration” can be used interchangeably, and the introduced gene can be exogenous and endogenous; wherein, after genetic modification, the highly expressed or overexpressed positive regulator gene or the lowly expressed or knocked out negative regulator gene can make the modified strain has significantly improved capacity for producing dibasic organic acid compared with its original strain.

Preferably, the expression product of the positive regulatory gene comprises one or more polypeptides of the invention or their derived polypeptides selected from the group consisting of aspartate aminotransferase, glutamic acid-aspartate transporter, C4-dicarboxylic acid transporter, malate dehydrogenase, pyruvate carboxylase, and glucose transporter. The expression product of the negative regulatory gene includes one or more polypeptides of the invention or their derived polypeptides selected from the group consisting of succinyl-CoA synthase, and malic acid-alpha ketoglutarate transporter.

More preferably, the polypeptides of the invention or their derived polypeptides are selected from the group consisting of

(I) sequences of SEQ ID NO.: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 26, or 96;

(II) polypeptides which are derived from (1) by one or more amino acids deleted, added or substituted from the sequence of SEQ ID NO.: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 26 or 96 and enable the engineered strain to have the capacity for producing dibasic organic acid; and

(III) polypeptides that have an amino acid sequence having a identity of ≥90% (preferably ≥95%, more preferably 98%) with the sequence of SEQ ID NO.: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 26, or 96 and enable the engineered strain to have a capacity for producing dibasic organic acid.

The derived polypeptide comprises a variant of a sequence as set forth by SEQ ID NO. 2, 4, 6, 8, 10, 12, 14, 16, 20, 22 or 96 which can make the original strain have the capacity for synthesizing the dibasic organic acid. These variants include (but not limited to): deletion, insertion and/or substitution of 1-3 (usually 1-2, preferably 1) of amino acids, and addition or deletion of one or more (usually less than 3, preferably less than 2, preferably less than 1) of amino acids at the C terminal and/or N terminal. For example, in this field, the function of proteins is usually not altered by the substitution of amino acids with close or similar properties. Also, for example, the addition or deletion of one or several amino acids at the C terminal and/or N terminal will not usually alter the structure and function of the protein. The terms “fragments”, “derivatives” and “analogues” refer to polypeptides that substantially maintain the ability to allow the original strain have the capacity for synthesizing the dibasic organic acid. The polypeptide fragments, derivatives or analogs of the invention may be (i) polypeptides having one or several conservative or non-conservative amino acid residues (preferably conservative amino acid residues) substituted, or (ii) polypeptides having substituent(s) in one or more amino acid residues, or (iii) polypeptides formed by fusing the polypeptide of the invention with another compound, such as a compound that extends the half-life of the polypeptide, or (iv) a polypeptide in which the additional amino acid sequence is fused to the polypeptide sequence (a fusion protein formed by fusion of a leader sequence, a secretory sequence, or a label sequence such as 6His). These fragments, derivatives and analogs are within the scope of what is known to those skilled in the art in light of the teachings herein. A preferred class of active derivatives refers to a polypeptide formed by up to 3, preferably up to 2, more preferably up to 1 of amino acid substituted with amino acids having close or similar property as compared to the amino acid residues of Formula 1. These conserved variant polypeptides are preferably produced by the amino acid substitutions according to Table 1.

TABLE 1 Preferred Primary residue Representative substitution substitution Ala (A) Val; Leu, Ile Val Arg (R) Lys; Gin; Asn Lys Asn (N) Gln, His; Lys; Arg Gln Asp (D) Glu Glu Cys (C) Ser Ser Gln (Q) Asn Asn Glu (E) Asp Asp Gly (G) Pro; Ala Ala His (H) Asn, Gin; Lys; Arg Arg Ile (I) Leu, Val; Met; Ala; Phe Leu Leu (L) Ile, Val; Met; Ala; Phe Ile Lys (K) Arg, Gln, Asn Arg Met (M) Leu, Phe, Ile Leu Phe (F) Leu, Val; Ile, Ala; Tyr Leu Pro (P) Ala Ala Ser (S) Thr Thr Thr (T) Ser Ser Trp (W) Tyr; Phe Tyr Tyr (Y) Trp, Phe, Thr, Ser Phe Val (V) Ile, Leu, Met; Phe, Ala Leu

In another preferred embodiment, a sequence of a polynucleotide encoding the polypeptide or its derived polypeptide of the invention (the polynucleotide of the invention) comprises:

(i) a polynucleotide encoding a sequence of SEQ ID NO.: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 26, 28, 30, or 96;

(ii) a polynucleotide having a sequence of SEQ ID NO.: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 25, or 95;

(iii) a polynucleotide sequence having an identity of ≥95% (preferably ≥98%) with a sequence of SEQ ID NO.: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 25, or 95; or

(iv) a polynucleotide derived from SEQ ID NO.: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 25, or 95 by 1-60 (preferably 1-30, more preferably 1-10) nucleotides deleted or added at 5′ end and/or 3′ end;

(v) a polynucleotide complementary to any one of the sequence of (I)-(IV).

The full-length sequence of the polynucleotide or its fragment of the invention can usually be obtained by PCR amplification, recombination or artificially synthesis. A preferred method to obtain the polynucleotides of the invention generally has the following steps:

(1) transforming or transducing a suitable host cell with a recombinant expression vector of a polynucleotide (or a variant) encoding the polypeptide of the invention, or a recombinant expression vector containing the polynucleotide;

(2) culturing the host cell in suitable medium

(3) isolating and purifying proteins from the medium or the cell.

The sequencesofthepolypeptidesoftheinventionandthecorrespondingecoding polynucleotides are shown in table 2:

TABLE 2 nucleotide amino acid Name of the Protein sequence sequence (polypeptide of the (SEQ ID (SEQ ID invention) source function NO.:) NO.:) succinyl-CoA Myceliophthora Negative 1 2 synthase thermophila regulation aspartate Myceliophthora Positive 3 4 aminotransferase thermophila regulation glutamic acid- Myceliophthora Positive 5 6 aspartate thermophila regulation transporter malic acid-alpha Myceliophthora Negative 7 8 ketoglutarate thermophila regulation transporter malate Myceliophthora Positive 9 10 dehydrogenase thermophila regulation C4-dicarboxylic acid Aspergillus Positive 11 12 transporter oryzae regulation C4-dicarboxylic acid Neurospora Positive 13 14 transporter crassa regulation C4-dicarboxylic acid Trichoderma Positive 15 16 transporter reesei regulation C4-dicarboxylic acid Myceliophthora Positive 17 18 transporter thermophila regulation C4-dicarboxylic acid Aspergillus Positive 19 20 transporter niger regulation C4-dicarboxylic acid Aspergillus Positive 21 22 transporter sojae regulation pyruvate carboxylase Aspergillus Positive 25 26 oryzae regulation glucose transporter Neurospora Positive 95 96 crassa regulation

Engineered Strain and Preparation Method Thereof

The terms “Engineering bacteria”, “engineered strain” and “genetic modified strain” of the invention can be used interchangeably, referring to the engineered strain in which the positive regulatory gene for synthesizing the dibasic organic acid is introduced or the expression of the positive regulatory gene for synthesizing the dibasic organic acid is up-regulated, and/or the expression of the negitive regulatory gene for synthesizing the dibasic organic acid is down-regulated. Among them, compared with its original strain, the dibasic organic acid producing capacity of the the engineered strain of the invention is significantly improved, wherein the dibasic organic acid comprises malic acid, succinic acid, fumaric acid, oxaloacetic acid, glutaric acid or adipic acid.

The original strain that can be modified to the engineered strain of the invention is usually filamentous fungi, especially filamentous fungi of Myceliophthora, such as Myceliophthora thermophila, Myceliophthora heterothallica, preferably Myceliophthora thermophila. The wild original strain usually does not have the capacity to synthesize the dibasic organic acid, or does not have the capacity for producing dibasic organic acid of an amount required in industry. Normally, an original strain in which a dibasic organic acid can be produced in the natural state, but will be rapidly converted into downstream metabolites (i.e., no accumulation of a dibasic organic acid can be formed) is also within the scope of the original strain of the invention. After genetic modification, the capacity for producing dibasic organic acid of the engineered strain of the invention is significantly improved, and the term “significantly improve” includes the strain that originally has no capacity for synthesizing the dibasic organic acid has that ability, or that ability is substantially increased compared with the original strain. Preferably, the term “significantly improve” means that compared to the original strain the capacity for producing dibasic organic acid of the engineered strain is increased or improved by at least 10%; preferably at least 10-50%; more preferably at least 50%-500%.

In addition, the original strain that can be modified into the engineered strain of the invention can also include Thielavia, preferably Thielavia terrestris; Aspergillus, preferably Aspergillus oryzae, Aspergillus flavus, Aspergillus sojae; and Rhizopus.

The engineered bacteria of the invention can be prepared by the following methods:

(a1) providing an expression vector carrying a positive regulatory gene for synthesizing the dibasic organic acid;

(b1) transferring the expression vector into a host cell;

(c1) culturing the host cell; and/or

the method comprises steps of:

(a2) knocking out the negitive regulatory gene for synthesizing the dibasic organic acid in host cells;

(b2) culturing the host cells;

wherein, the host cell is the original strain.

The negative regulatory gene of the invention can be knocked out or down regulated by genetic engineering means or substances that inhibit the expression and/or activity of the negative regulatory gene so as to obtain a new transgenic engineered bacterium. Such substances are known as “inhibitors of the invention” or “negative regulatory genes inhibitors”. The inhibitors, for example, include antibodies inhibitory mRNA, antisense RNA, microRNA (miRNA), siRNA, shRNA to the negative regulatory genes, and activity inhibitors of zinc finger transcription factors. A preferred inhibitor is a negative regulatory gene of siRNA, such as a sequence of SEQ ID NO.: 1. According to the sequence of SEQ ID NO.: 1 of the invention, siRNAs that inhibiting its expression can be designed by conventional techniques in the art, and the preferred siRNAs are shown in SEQ ID NO.: 74 and 75.

Combination of the Regulatory Gene for Producing Dibasic Organic Acid or the Expression Products Thereof

The invention also provides a combination of the polypeptides of the invention or their encoding polynucleotides. The experiment has proved that it can effectively improve the capacity of the strain for producing dibasic organic acid by using the combination of the invention to modify the original strain simultaneously. Wherein the combination of the regulatory gene expression products of the invention may include at least two polypeptides selected from the group consisting of

(Ia) a sequence of SEQ ID NO.: 4, 6, or 10 or combinations thereof; or

(IIa) a polypeptide derived from (Ia) by one or more amino acids deleted, added or substituted from the sequence of SEQ ID NO.: 4, 6, or 10 and enabling Myceliophthora strains to have the capacity for producing dibasic organic acid and/or increasing the capacity for producing dibasic organic acid; and optional

(Ib) a sequence of SEQ ID NO.: 12, 14, 16, 18, 20, 22, 26, 28, 30, 96 or combinations thereof;

(IIb) a polypeptide derived from (Ib) with one or more amino acids deleted, added or substituted from the sequence of SEQ ID NO.: 12, 14, 16, 18, 20, 22, 26, 28, 30, or 96 and enabling Myceliophthora strains to have the capacity for producing dibasic organic acid and/or increase the capacity for producing the dibasic organic acid.

While the combination of the dibasic organic acid producing regulatory genes of the invention contains at least two polynucleotides, and the polynucleotides correspondingly encode the polypeptides in the combination of the expression products of the invention, respectively.

In addition, the invention also provides a vector containing the gene combination of the invention, and a host cell comprising the vector or the chromosome with the positive regulator gene for producing the dibasic organic acid intergrated and/or with the negative regulator gene for producing the negative regulator gene down-regulated.

Preferrably, the chromosome of the host cell of the invention is artificially integrated with the polynucleotide that encodes the polypeptides of SEQ ID NO.: 4, 6, and/or 10; or the gene that encodes the polypeptides of SEQ ID NO.: 2 and/or 8 in the chromosome of the host cell is knocked out or weakened; and optionally

the chromosome of the host cell is integrated with one or more polynucleotides selected from the polypeptides of SEQ ID NO.: 4, 6, 10, 12, 14, 16, 18, 20, 22, 26, and 96.

The Beneficial Effects of the Invention

(a) By using a protoplast or an agrobacterium mediated transformation/transfection method, a heterologous or homologous nucleic acid sequence is stably introduced into an original strain, said nucleic acid sequence is operably linked to the expression regulatory region, while knocking-out or mutation or attenuated expression and screening for gene and knock out the transformants with the assistance of green fluorescent protein are also included. The genetic operation technique system of Myceliophthora thermophila is underdeveloped. The present invention firstly uses the genetic engineering technology to transform Myceliophthora thermophila and thereby producing the dicarboxylic acid by fermentation.

(b) A set of new key genes impacting the dicarboxylic acid fermentation level in filamentous fungi are discovered, and the fermentation level of the dicarboxylic acid (especially malic acid) is improved through the genetic modification in Myceliophthora, wherein the genes include succinyl-CoA synthase, aspartate aminotransferase, malic acid-alpha ketone glutaric acid transporter, glutamic acid-aspartate transporter and C4-dicarboxylic acid transporter, glucose transporter, or malate dehydrogenase.

(c) Under certain conditions, dibasic acids are produced from the recombinant strains with a variety of carbon sources as fermentation substrates, including biomass resources, which greatly reduces the cost of fermentation of biomass-based chemicals. L-malic acid is directly produced by fermentation of microorganisms (and the production of other dibasic organic acids and even more chemicals are allowed).

(d) The fermentation temperature is high. The fermentation can be conducted at 40-50 degrees (preferably 45 degrees), which significantly save the condensation costs during the fermentation, thereby reducing the cost of the fermentation. The strains of the invention can synthesize the dibasic organic acid with high yield at high temperature which can not be tolerated by the room temperature filamentous fungi (such as Aspergillus).

The invention will now be further described with reference to specific embodiments. It should be understood that these examples are merely illustrative of the invention and are not intended to limit the scope of the invention. The experimental methods not specified for conditions in the following examples are generally carried out according to conventional conditions, such as those described in Sambrook et al., Molecular Cloning: Laboratory Manual (New York: Cold Spring Harbor Laboratory Press, 1989), or conditions suggested by the manufacturer. Unless otherwise specified, percentages and parts are those by weight.

Example 1 Overexpressing C4-Dicarboxylic Acid Transporter Encoding Gene Mae in Myceliophthora thermophila to Allow Myceliophthora thermophila to Obtain a Capacity for Producing Malic Acid

1. Construction of a vector with mae overexpressed (pAN52-mae)

Pan52-TB-Intron (Liu Q, Li J, Ying S, Wang J, Sun W, Tian C, Feng M. 2014. Unveiling equal importance of two 14-3-3 proteins for morphogenesis, conidiation, stress tolerance and virulence of an insect pathogen. Environ Microbiol. doi: 10.1111/1462-2920.12634) was used as the skeleton to construct the expression vector, wherein the plasmid pCSN44 (available from fungal genetics stock center) was used as a template and Hygromycin phosphotransferase coding gene (hph) was PCR amplified under the regulatory of TrpC promoter according to the guidance of primers. The sequences of the primers are as follows:

hph-F (SEQ ID NO.: 23): GCTCTAGACAGAAGATGATATTGAAGGAGC hph-R (SEQ ID NO.: 24): CCCAAGCTTCTATTCCTTTGCCCTCGGACGAG

The hph.PCR reaction system is:

5× phusion HF buffer 10 μL, 10 mM dNTPs 1 μL, GLT-F 2.5 μL, GLT-R 2.5 μL, cDNA 1 μL, Phusion DNA polymerase 0.5 μL, water 32.5 μL. The PCR reaction conditions are as follows: 98° C. for 30 s at first, then 98° C. for 10 s, 65° C. for 30 s, 72° C. for 1.5 min, 34 cycles, 72° C. for 10 min, and 4° C. for 10 min at last.

After PCR reaction, the gene was digested by XbaI and HindIII and ligated into the linearized vector pAN52-TB-Intron, which was digested by the same enzymes. The ligation product was digested with restriction endonuclease and subjected to sequencing. The sequence result shows that the nucleotide sequence of hph is shown as SEQ ID NO.: 27, which indicates that the recombinant expression plasmid carrying hph gene with correct sequence and insertion cite was obtained and named as pAN52-hph.

C4-dicarboxylic acid transporter encoding gene mae (XM_001820829.2, SEQ ID NO.: 11) was obtained by PCR amplication taking Aspergillus oryzae DSM1863 (DSMZ, purchased from German Microorganism and Cell Culture Co., Ltd.) as the template under the guidance of primers, and the gene was digested by BgIII and then ligated into linearized vector pAN52-hph which was digested by BgIII and EcoRV, the ligation product was digested with restriction endonuclease, and a vector carrying mae gene was obtained and named as pAN52-hph-mae. and the sequences of the primers are as follows:

Mae-F: 5′(SEQ ID NO.: 43): GGAAGATCTTAATTAACTCGAGCGGCCGCGTTTAAACACTAGTATGCTG ACACCTCCCAAGTTTG Mae-R: 5′(SEQ ID NO.: 44): ATCCTAATCAGATACAT CCTCATCTTTA

Taking the genes of original strain Myceliophthora thermophila ATCC42464 (purchased from American Type Culture Collection) as the template, a promoter of 1.4 kb upstream of translation extension factor encoding reading frame (MYCTH_2298136) (named as Ptef promoter) was amplified by PCR and the sequence of SEQ ID NO.: 28 was amplified using primers, and the sequences of the primers are as follows:

Tef-F: (SEQ ID NO.: 29): CCTTAATTAACATGTACCTTGACGTCCTCCGAG Tef-R: (SEQ ID NO.: 30): GGACTAGTTCTGAAGAACGAAACTGGC GACT

After PCR reaction, the gene was digested by PacI and SpeI and ligated into the linearized vector pAN52-hph-mae, which was digested by the same enzyme. The ligation product was digested with restriction endonuclease and verified to obtain a mae expression vector under the regulatory of promotor tef which and named as pAN52-mae.

The physical spectrum is shown in FIG. 1.

2, Introducing the Expression Vector (pAN52-Mae) into Myceliophthora thermophila

2.1 Myceliophthora thermophila ATCC42464 was cultured in MM slant medium [20 mL of 50× Vogel's salt, 20 g of sucrose, 15 g of agar, and 20 mL of histidine (50 mg/mL) were adjusted to 1 L and sterilized under high pressure. 50× Vogel's salt (1 L): trisodium citrate (½H₂O) 150 g, anhydrous KH₂PO₄ 250 g, anhydrous NH₄NO₃ 100 g, MgSO₄.7H₂O 10 g, CaCl₂.2H₂O 5 g, salt solution of trace elements 5 mL and biotin (0.1 mg/mL) 2.5 mL were adjusted to 1 L] for 10 days at 45° C. to be used.

2.2 Protoplast Transformation of Myceliophthora thermophila

1) Preparation of the Mycelium

Mature spores of Myceliophthora thermophila was collected with sterilized water containing 0.05% Tween-80. The hyphae was filtered and obtained with the lense paper and then placed on MM plates with cellophane and cultured at 45 for 14 h.

2) Preparation of the Protoplast

The cellophane with mycelium was placed in 30 mL of lysate (formulation: 0.15 g of lyase, and 30 mL of solution A was added aseptically, the product was filtered for sterilization, solution A: 1.0361 g of potassium dihydrogen phosphate and 21.864 g of sorbitol were dissolved in 90 mL of deionized water. Potassium hydroxide was used to adjust the pH to 5.6, and the product was quantified to 100 mL and sterilized at high temperature) and lysed at 28° C. for 2 h and gently shaked every 20 min.

Then the mixture was filtered with cellophane and centrifuged at 4° C. under 2000 rpm for 10 min. The supernatant was discarded. To the remain was added 4 mL of solution B (0.735 g of calcium chloride, 18.22 g of sorbitol and 1 mL of Tris-HCl 1M pH 7.5 was dissolved in 90 mL of deionized water. Hydrochloric acid was used to adjusted the pH to 7.6, and the product was quantified to 100 mL and sterilized at high temperature). The mixture was centrifuged at 4° C. under 2000 rpm for 10 min. The supernatant was discarded. A certain volume of solution B was added as 200 L/plasmid.

3) Transformation of Protoplast

To the pre-cooled 15 mL centrifuge tubes are sequentially added with 50 μL of pre-cooled PEG (12.5 g of PEG6000, 0.368 g of CaCl₂, 500 μL of Tris HCl, 1M, pH, 7.5), 10 μL of plasmid pAN52-mae linearized with HindIII, and 200 μL of protoplasts. After placing on ice for 20 min, to the centrifuge tubes were added with 2 mL of precooled PEG, and the product was kept in room temperature for 5 min, then added with 4 mL of solution B and gently mixed. 3 mL of the above solution was added into 12 mL of melt MM medium with corresponding antibiotics. The mixture was placed in the plate, and cultured at 45° C. After 2 d-4 d, single mycelium was selected from the solution under the stereomicroscope and subjected to the corresponding resistant plate for growth.

2.3 Validation of Myceliophthora thermophila Transformant

1) Genome Extraction

Genomic DNAs were extracted from the transformants selected in the above transformation process, using phenol chloroform method, which includes the following operations:

1) To 2.0 mL sterile DNA extraction tube was added 200 mg of zirconium beads and 1 mL of lysate (lysis buffer, formula: 0.2M Tris-HCl (pH 7.5), 0.5M NaCl, 10 mM EDTA, 1% SDS (w/v)). Myceliophthora thermophila hyphae growing on the plate were selected and placed in DNA extraction tube.

2) All the DNA extraction tubes were placed on the grinding mill, which were vibrated at the maximum speed for 30 s twice.

3) The tubes were heated in water bath at 65° C. for 30 min, and during the process the tubes were taken out for vortex oscillation every few minutes.

4) The tubes were retrieved after the water bath process completed, and to each tube was added 80 μL of pH 7.5, 1M, Tris HCl for neutralization.

5) 400 μl of phenol was added to the tubes: chloroform (1:1), and centrifuged at 13000 rpm for 5 minutes.

6) 300 μL of supernatant was taken and placed into a new 1.5 mL EP tube, to which was added 600 μL of 95% ethanol (DNA grade).

7) The product was incubated on ice for one hour, followed by centrifugation at 4° C. at 13000 rpm, then a white DNA precipitate can be observed at the EP tube bottom.

8) 400 μL of 75% alcohol (DNA level) was used for washing, and the product was centrifuged at 4 degree at 13000 rpm, and the supernatant was gently removed.

9) The EP tube was subjected to a vacuum concentrator and vacuum dried to remove alcohol.

10) 50 μL of ddH₂O was added to dissolve DNA, and NanoDrop was used to measure DNA concentration. After the measurement, the extracted DNA was put in a refrigerator at −20° C. for PCR verification in the next step.

2) PCR Verification of Myceliophthora thermophila Transformat

The extracted genomic DNAs were used as a template to validate the transformants with tef-F and mea-R as primers. PCR reaction system: 5× phusion GC buffer 4 μL, 10 mM dNTPs 0.2 μL, 1 μL for each primers, genome 1 μL, DMSO 0.6 μL, Phusion DNA polymerase 0.1 μL, and water 12.1 μL. PCR reaction condition: 98° C. for 30 s at first, then 98° C. for 10 s, 62° C. for 30 s, 72° C. for 1.5 min, 30 cycles, 72° C. for 10 min and 4° C. for 10 min at last.

3) PCR amplification products were subjected to 1% agarose gel electrophoresis (110 V voltage, 30 min). Under the gel imaging system, the gene amplified bands were observed and showed that in the guidance of the upstream primer tef-F and the downstream primer mae-R, a 2360 bp target band was obtained by PCR amplification, which indicated that the pAN52-mae linearized by hindIII was integrated into the Myceliophthora thermophila genome.

3. Determination of Malic Acid Producing Capacity of Myceliophthora thermophila Transformant

The above verified transformants were all inoculated into 50 mL of medium with crystalline cellulose (Avicel) as the carbon source in a 250 mL Erlenmeyer flask (formulation: 75 g/L of carbon source, 6.0 g/L of peptone, 0.15 g/L of KH₂PO₄, 0.15 g/L of K₂HPO₄, 0.10 g/L of CaCl₂.2H₂O, 0.10 g/L of MgSO₄,7H₂O, 80.0 g/L of calcium carbonate, 1 mL/L 0.5 g/L biotin, and 1 ml/L of trace element solution; the formulation of the trace element solution (100 mL): 5 g of C₆H₈O.7H₂O, 5 g of ZnSO₄.7H₂O, 1 g of Fe(NH₄)₂(SO₄).6H₂O, 0.25 g of CuSO₄.5H₂O, 0.05 g of MnSO₄.H₂O, 0.05 g of H₃BO₃, 0.05 g of NaMoO₄.2H₂O, dissolved in water to a volume of 100 mL) in an inoculation amount of 2.5×10⁵/mL and cultured at 45° C. in 150 rpm. On the eighth day the malic acid content was determined.

1) Sample Treatment:

1 mL of fermentation broth was placed in a 15 mL centrifuge tube, to which was added 1 mL of 1M H₂SO₄, and then placed at 80° C. for 30 min with full vibration every 0 min. After that 2 mL double distilled water was added to the centrifuge tube. After through vibration, 1 mL of liquid was subjected to a 1.5 mL centrifuge tube and centrifuged in 12000 rpm for 10 min. The content of C4-bicarboxylic acid was verified by measuring the supernatant.

2) Verification of the Content of C4-Bicarboxylic Acid

The content of malic acid and succinic acid in the treated sample were determined by HPLC, wherein the detector was UV detector, the mobile phase was 5 mM H₂SO₄, and the flow rate was 0.5 mL/min. The results showed that mae overexpression in Myceliophthora thermophila can significantly promote the malic acid production, in which the strains with a highest yield were named as JG141. On the eighth day, the malic acid production was 42 g/L (FIG. 9), the succinic acid production was 3.86 g/L for the corresponding carbon source. The experiments showed that after the genetic modification, the fermentation of malic acid could be carried out in Myceliophthora thermophila by using the carbon source including crystalline cellulose.

Example 2 Overexpression of C4-Dicarboxylic Acid Transporter Encoding the Genes of Myceliophthora thermophila from Different Origins can Obtain Recombinant Microorganisms with Significant Increased Capacity for Producing Malic Acid

1. Homology Comparison Analysis of C4-Dicarboxylic Acid Transporter

In this example, C4-dicarboxylic acid transporter from Aspergillus oryzae NRRL3488 (AO090023000318, mae, SEQ ID NO.: 12), C4-dicarboxylic acid transporter from Neurospora crassa (XP_958365, NCmae, SEQ ID NO.: 14), C4-dicarboxylic acid transporter from Trichoderma reesei (XP_006963989, Trmae, SEQ ID NO.: 16), C4-dicarboxylic acid transporter from Myceliophthora thermophila (XP_003663832, Mtmae, SEQ ID NO.: 18), C4-dicarboxylic acid transporter from Aspergillus niger NRRL599 (XM_001398094, Anmae, SEQ ID NO.: 20), C4-dicarboxylic acid transporter from Aspergillus sojae NBRC4239 (Asmae, SEQ ID NO.: 22) were selected.

2. Construction of C4-Dicarboxylic Acid Transporter Gene Expression Vector Promoter

A 1.0 kb upstream promoter of the translation elongation encoding frame tef (MYCTH_2298136) was amplified by PCR from the original strain of Myceliophthora thermophila ATCC 42464 as a template. The reaction system and conditions were shown in Step 1 of Example 1. According to the constructed plasmids, the primers used for PCR amplification were:

Tef-2F: (SEQ ID NO.: 55) GAAGATCTCATGTACCTTGACGTCCTCCGAG Tef-2R: (SEQ ID NO.: 56) GGACTAGTTCTGAAGAACGAAACTGGCGACT

After PCR reaction, the recombinant plasmid was digested with BgIII and SpeI and ligated into the linearized vector pAN52-TB-Intron, which was digested by the same enzyme. The ligation product was digested with restriction endonuclease and identified to obtain the recombinant vector, named as pAN52-TB-Ptef. The physical map was shown in FIG. 2.

3. Construction of C4-Dicarboxylic Acid Transporter Gene Expression Vector

3.1 The gene encoding C4-dicarboxylic acid transporter Ncmae (SEQ ID NO.: 13) was obtained by PCR amplification using Neurospora crassa (purchased from FGSC) as a template, and the primers used for PCR amplification were as follows:

NCmae-F: (SEQ ID NO.: 45) GGACTAGTATGGGCAGCCAGCCTCCCATGC NCmae-R: (SEQ ID NO.: 46) CGGAATTCCTAATGATCCTCCACATCCTCA

3.2 The gene encoding C4-dicarboxylic acid transporter Trma(SEQ ID NO.: 15) was obtained by PCR amplification using Trichoderma reesei (purchased from ATCC) as a template, and the primers used for PCR amplification were as follows:

Trmae-F: (SEQ ID NO.: 47) GGACTAGTATGAAAGCGGCATTCCCTCATGC Trmae-R: (SEQ ID NO.: 48) CGGAATTCTCAGTGATCCTCCACATTCTCATC

3.3 The gene encoding C4-dicarboxylic acid transporter Mtmae(SEQ ID NO.: 17) was obtained by PCR amplification using Myceliophthora thermophila ATCC42464 (purchased from ATCC) as a template, and the primers used for PCR amplification were as follows:

Mtmae-F: (SEQ ID NO.: 49) CGGACTAGTATGTCAACACCGCGGCGAAG Mtmae-R: (SEQ ID NO.: 50) CCGGAATTCTTAATGATCCTCCACGTCCTC

3.4 The gene encoding C4-dicarboxylic acid transporter Anmae(XM_001398094)(SEQ ID NO.: 19) was obtained by PCR amplification using Aspergillus niger NRRL599 as a template, and the primers used for PCR amplification were as follows:

Anmae-F: (SEQ ID NO.: 51) GGACTAGTATGAACGTTGAAACGAGC Anmae-R: (SEQ ID NO.: 52) CGGAATTCTCATTCAGACACATCCTCAT

3.5 The gene encoding C4-dicarboxylic acid transporter Asmae(SEQ ID NO.: 21) was obtained by PCR amplification using Aspergillus sojae NBRC4239 as a template, and the primers used for PCR amplification were as follows:

Asmae-F: (SEQ ID NO.: 53) GCTCTAGAATGCTGACACCTCCCAAGTTTGAGGATG Asmae-R (SEQ ID NO.: 54) CCTTAATTAACTAATCAGATACATCCTCATCTTTACCC

The PCR product of C4-dicarboxylic acid transporter gene fragments obtained through PCR amplification and analysis and the plasmid pAN52EF-Intron were digested by restriction endonuclease SpeI and EcoRI. Then, they were ligated by T4 DNA ligase to obtain the expression plasmids, named as pAN52-Ptef-Ncmae, pAN52-Ptef-Trmae, pAN52-Ptef-Mtmae, pAN52-Ptef-Anmae, and pAN52-Ptef-Asmae respectively.

4. Analysis of Malic Acid Produced by Fermentation of Myceliophthora thermophila Recombinant Transformants

(1) The Obtain of Recombinant Myceliophthora thermophila Transformants

The constructed gene expression vectors (pAN52-Ptef-Ncmae, pAN52-Ptef-Trmae, pAN52-Ptef-Mtmae, pAN52-Ptef-Anmae, pAN52-Ptef-Asmae) were integrated into the original strains of Myceliophthora thermophila strains genome and use glufosinate at a final concentration of 100 μg/mL as an antibiotic for screening. The method of the example was shown in step 2 in example 1. The transformat was verified using primer tef-2F and the downstream primer corresponding to gene cloning. PCR system and methods were shown in step 1.3 in example 1.

All of the transformants verified were inoculated into a 250 mL conical flask containing 50 mL of medium with crystalline cellulose (Avicel) as the carbon source (see step 3 in example 1) in a inoculation amount of 2.5×10⁵/mL, then subjected to the culture at 45° C. in 150 rpm, and the sample was taken on eighth day. After the sample was treated by the method as described in step 3.2 of example 1, the malic acid content in the fermentation broth was determined.

The results showed that overexpression of C4-dicarboxylic acid transporter derived from different species in Myceliophthora thermophila can significantly promote the production of malic acid, and the malic acid yield went up to (see FIG. 10): 37.9 g/L (Ncmae), 26.1 g/L (Mtmae), 16.6 g/L (Trmae), 0.24 g/L (Anmae) and 59.4 g/L (Asmae), respectively. It showed that C4-dicarboxylic acid transporter from Neurospora crassa, Myceliophthora thermophila, Trichoderma reesei and Aspergillus sojae can be used to construct Myceliophthora thermophila strains for malic acid fermentation in industry. But it should be noted that although C4-dicarboxylic acid transporters from Aspergillus niger and Aspergillus oryzae have a high identity (about 90%), in this experiment when the C4-dicarboxylic acid transporter from Aspergillus niger was overexpressed in the Myceliophthora thermophila, the transformants did not show the capacity of producing malic acid suitable for industrial application. It can be seen that even in the Aspergillus strain with an ability for accumulating malic acid, the proteins with high identity failed to enable other strains have better malic acid-producing capacity.

For the genes derived from different strains, the inventors conducted more experiments to study whether genes from non-dominant strains for organic acids accumulation (Neurospora crassa, Trichoderma reesei, etc.) can be used by metabolic engineering method to improve the capacity for producing dibasic acid of their own or other strains.

Example 3 the C4-Two Carboxylic Acid Transport Protein Coding Gene Mae and Pyruvate Carboxylase Gene Pyc are Simultaneously Expressed in the Myceliophthora thermophila so as to Strengthen the Ability to Produce Malic Acid

1. Construction of Co-Expression Vectors Containing Mae and Pyc

The promoters of of Aspergillus nidulans gpdA was amplified by PCR using the plasmid pAN52-TB-Intron as the template in the guidance of primers. The PCR conditions and system were described in step 1 of Example 1. The primer was shown as follows:

ANgpadA-F: (SEQ ID NO.: 61) CCTTAATTAAGTCCAGATCATGGTTGACCGGTG ANgpdA-R: (SEQ ID NO.: 62) GAACCTCCTTCAGAGAGGTTCGTGTTTAAACTGATGTCTGCTCAAGCGG GGTA

Then the cellobioses hydrolase encoding gene cbh (MYCTH_109566) terminator (SEQ ID NO.: 85) was amplified by PCR using the primers and the original strain Myceliophthora thermophila genome as a template. The primers were shown as follows:

CBH-F: (SEQ ID NO.: 63) ACCCCGCTTGAGCAGACATCAGTTTAAACACGAACCTCTCTGAAGGAGG TTC CBH-R: (SEQ ID NO.: 64) CCCAAGCTTCTAATAGGGATAATAAGCTAGGGTC

The gpdA promoter and cbh terminator were ligated together by fusion PCR using the method of gene overlap extension (SOE), which was invented by Horton et al. 1989 (Horton R M, Hunt H D, Ho S N, Pullen J K, Pease L R. 1989. Engineering hybrid genes without the use of restriction enzymes: gene splicing-by-overlap extension. Gene 77: 61-68).

The An_gpdA promoter and cbh terminator were digested by HindIII to provide adhesive ends and then ligated into pAN52-mae which was digested with the same enzyme to give the recombinant vector: pAN52-mae-PgpdA-Tcbh.

Pyruvate carboxylase encoding gene pyc (XM_001820829.2, SEQ ID NO.: 25) was amplified by PCR using Aspergillus oryzae DSM1863 cDNA as a template in the guidance of primer PYC-F (SEQ ID NO.: 57) and PYC-R (SEQ ID NO.: 58), then digested by PmeI and ligated into pAN52-mae-PgpdA-Tcbh which was digested with the same enzyme. The recombinant plasmid was verified by PCR with primers PYC-F and PYC-R, and then sequenced. The sequencing results confirmed that the sequence of the plasmid was the nucleotide sequence of pyc gene, which showed that the recombinant plasmid with the correct insertion position and carrying pyc gene was obtained and named as pAN52-mae-pyc. The physical map of the expression vector was shown in FIG. 3.

2. Determination of Malic Acid-Producing Capacity by Myceliophthora thermophila Transformants Fermentation Using Monosaccharides, Glycans and Biomass as a Carbon Source.

The coexpression vector pAN52-mae-pyc containing mae and pyc was linearized by BgIII and then integrated into the genome of the original strain Myceliophthora thermophila, and the method was described in step 2 of example 1. The transformants were obtained and verified using peimers mae-F (SEQ ID NO.: 43) and mae-R (SEQ ID NO.: 44) (to verify mae was intergrated into the genome), PYC-F and PYC-R (to verify pyc was integrated into the genome). PCR system and conditions was shown in step 1.3 of example 1.

The verified transformants were all inoculated into 250 mL Erlenmeyer flask with 50 mL of medium containing glucose, D-xylose, cellobiose, xylan, crystalline cellulose, sucrose, soluble starch, corncobs xylose slag and corncobs delignification as carbon source (the formulation was shown in step 3 of example 1) and cultured in an inoculated amount of 2.5×10⁵ cells/mL at 45° C. in 150 rpm and sampled on the eighth day. After the sample was treated by the method as described in step 3.2 of example 1, the malic acid content in the fermentation broth was determined.

The results showed that when mae and pyc were overexpressed simultaneously in Myceliophthora thermophila, the production of malic acid was significantly promoted. One of the strains was named as JG207. On the eighth day, the yields of malic acid and succinic acid in the transformants using various carbon sources were: 62 g/L and 3.2 g/L (glucose), 28 g/L and 6.4 g/L (D-xylose), 78.7 g/L and 8.6 g/L (cellobiose), 61.3 g/L and 11 g/L (xylan), 63 g/L and 7.2 g/L (crystalline cellulose), 36.3 g/L and 4.7 g/L (sucrose), 46.3 g/L and 16.0 g/L (soluble starch), 36.8 g/L and 9.1 g/L (corncobs xylose slag), 55.15 g/L and 8.1 g/L (corncob delignification slag).

Example 4 Overexpressing Malate Dehydrogenase Encoding Gene Mdh in Myceliophthora thermophila Transformant Further Enhanced its Ability to Produce Malic Acid

1. Construction of Mdh Overexpression Vectors

The promoter PtrpC (SEQ ID NO.: 86) of the tryptophan synthase-encoding gene from Aspergillus nidulans was amplified under the guidance of primers using pAN52-TB-Intron as a template. The primers were shown as follows:

Trpc-F: (SEQ ID NO.: 65) CTTTCTAGACGACGTTAACTGATATTGAAGGAGC Trpc-R: (SEQ ID NO.: 66) CGTGCAATCCATCTTGTTCAATCATTTGGATGCTTGGGTAGAATAGGTAA

The neomycin phosphotransferase encoding gene neo (GI: 339515868) was amplified by PCR using primers and plasmid pEGFP-N2 as a template. The reaction system and conditions were shown in Step 1 of Example 1. The primers were shown as follows:

NEO-F: (SEQ ID NO.: 67) TTACCTATTCTACCCAAGCATCCAAATGATTGAACAAGATGGATTGCACG NEO-R: (SEQ ID NO.: 68) AAAAAAAGCTTGGTACCATCGATGCGGCCGCCCGCGGTCAGAAGAACTCG TCAA.

The sequence of PtrpC and Neo were ligated together by the fusion PCR method and the specific method is gene overlap extension (SOE).

PtrpC and neo were digested by XbaI and HindIII to obtain sticky ends and then ligated into pAN52-TN-Intron which was digested by the same enzymes to obtain the recombinant vector with Neo as the screening marker, and the product was named as pAN52-TN.

The sequence of the promoter MtPgpdA at 1.5K upstream of 3-phosphoglyceraldehyde dehydrogenase-encoding gene of the original strain Myceliophthora thermophila was optimized to remove the restriction site, and the artificially synthesized sequence was shown as SEQ ID NO.: 69. Using the above as a template and in guidance of primers, MtPgpdA was amplified, then digested by BgIII and BamHI, and then ligated into the linearized vector pAN52-TN which was digested by the same enzyme enzymes, so as to obtain the recombinant plasmid containing gpdA promoter: pAN52-TN-MtPgpdA. The primers were shown as follows:

MtPgpdA-F: (SEQ ID NO.: 70) TGCAGATCTTTAATTAACTCGAGTGACGGTGCTTTTCACCTCTC MtPgpdA-R: (SEQ ID NO.: 71) AGTGGATCCGAATTCGATATCGTTTAAACACTAGTTTTGATTTC TGTGATGTGG

The malate dehydrogenase encoding gene mdh (MYCTH_2315052) in Myceliophthora thermophila was amplified by PCR in the guidance of primers and using the original strain of Myceliophthora thermophila cDNA as template. The primers were shown as follows:

MtMDH-F: (SEQ ID NO.: 59) CGGACTAGTATGGTCAAAGCTGTCGTTGCTG MtMDH-R: (SEQ ID NO.: 60) CGCGGATCCTCACTTCTGGGGGGGGTTGTG.

After digested by SpeI and BamHI, the plasmid was ligated into linearized plasmid pAN52-TN-MtPgpdA which was digested by the same enzymes so as to obtain recombinant vector expressing mdh, named as pAN52-mdh, and the physical map of the expressing vector was shown in FIG. 4.

2. Determination of the Capacity of Myceliophthora thermophila Transformant for Producing Malic Acid.

The mdh overexpression vector pAN52-mdh was linearized by BgI II and then intergrated into the Myceliophthora thermophila JG207 strains genome. The final concentration was 100 μg/mL. G418 was used as the screening antibiotics. The method was shown in step 2 of example 1. The transformant was verified and contained using primers MtPgpdA-F and MtMDH-R. The PCR system and methods were described in step 1.3 of example 1.

All of the transformants verified were inoculated into a 250 mL conical flask containing 50 mL of medium with crystalline cellulose (Avicel) as the carbon source (see step 3 in example 1) in a inoculation amount of 2.5×10⁵/mL and subjected to culture at 45° C. in 150 rpm, and sampled on eighth day. After the sample was treated by the method as described in step 3.2 of example 1, determine the malic acid content in the fermentation broth.

The results showed that when mae and pyc were overexpressed in Myceliophthora thermophila simultaneously, the malic acid production can be significantly promoted. One of the strains was named as JG319. On the eighth day the yield of malic acid was 75 g/L (FIG. 9), the yield of succinic acid was 9.3 g/L. The conversion rate of malic acid went up to 1.0 g/g Avicel.

Example 5 Inhibiting the Expression of Succinyl-CoA Synthase by RNA Interference to Improve the Fermentation Level of Malic Acid

1. The upstream promoters interfering vector construction, named as P1 and P2 promoter (SEQ ID NO.: 72 and 73), were digested with BgIII and PmeI and then ligated respectively into linearized vector pAN52-TB-Intron which was digested with the same enzymes to obtain the recombinant plasmid respectively named as pAN52-TB-Psilent-A and pAN52-TB-Psilent-B.

The first interference sequence SCL-S1 (SEQ ID NO. 74) of the succinyl-CoA synthase encoding gene scl in Myceliophthora thermophila was amplified by PCR in the guidance of primers, and the primers were shown as follows:

SCL1-F: (SEQ ID NO.: 31) CCATCGATCATCAAGAACCTGTACCGCATC SCL1-R:, (SEQ ID NO.: 32) GGGTTTAAACCAATGATGGGGA, TCTTCAGGTC.

The second interference sequence SCL-S2 (SEQ ID NO.: 75) of the succinyl-CoA synthase encoding gene scl in Myceliophthora thermophila was amplified by PCR in the guidance of primers.

SCL2-F: (SEQ ID NO.: 33) CGCGGATCCCAATGATGGGGATCTTCAGGTC SCL2-R: (SEQ ID NO.: 34) CGCGGATCCGTTTAAACCATCAAGAACCTGTACCGCATC. 

Two interference sequences of scl were digested with ClaI/PmeI and BamHI respectively and then ligated into the linearized plasmids pAN52-TB-Psilent-A and pAN52-TB-Psilent-B which were digested with the same enzymes so as to obtain a binary vector of transcription element containing SCL gene interference sequence hairpin structure and screening marker bar gene: pAN52-SCLsilent-A and pAN52-SCLsilent-B. The physical maps were shown in FIG. 5 and FIG. 6.

2, Interfering the Succinyl-CoA Synthase Expression Significantly Increased the Malic Acid-Producing Ability in Microorganisms

The binary vector pAN52-SCLsilent-A and pAN52-SCLsilent-B of transcription element containing the hairpin structure of the SCL gene interference sequence and screening marker bar gene were integrated into the genome of the Myceliophthora thermophila JG207 strain respectively. The final concentration was 100 μg/ML and glufosinate was used as screening antibiotic. The method was described in Example 1, Step 2. The transformant was obtained and verified using primers Intron-F(AGCTGTTTACTCATTATTAC, SEQ ID NO.: 76) and SCL2-R(SEQ ID NO.: 34). The PCR system and methods were described in step 1.3 of example 1.

All of the verified transformants were inoculated into a 250 mL conical flask containing 50 mL of medium with crystalline cellulose (Avicel) as the carbon source (see step 3 in example 1) at a inoculation amount of 2.5×10⁵/mL. The product was cultured at 45° C. in 150 rpm, and sampled on eighth day. After the sample was treated by the method as described in step 3.2 of example 1, the malic acid content in the fermentation broth was determined.

The results showed that the yield of the malic acid in Myceliophthora thermophila JG207 intergrated with pAN52-SCLsilent-A was similar to that in the original strain JG207, and the yield of malic aid was 68 g/L on the eighth day. While compared to the original strain JG207, the yield of the malic acid in Myceliophthora thermophila JG207 intergrated with pAN52-SCLsilent-B was improved significantly, the strain with highest yield was named as JG207S, and the final yield of the malic acid (fermented for eighth days) was 74.8 g/L (FIG. 9), which was increased by 15.3%.

This example illustrates that the transcription of the RNA interference sequence hairpin structure regulated by the time-controlled promoter interfered the translation of the key enzymes encoding genes in the TCA cycle, thereby reducing the tricarboxylic acid cycle and significantly improving the malic acid producing ability in the microorganism.

Since then, the inventors used the single gene mutant of Neurospora crassa as a host to screen out the new key genes for producing malic acid in microorganism: aspartate aminotransferase, glutamic acid-aspartate transporter, Malic acid-alpha ketoglutarate transporter. In the following experiment, the inventors further verified these newly discovered genes which were related to the synthesis of dicarboxylic acid.

Example 6 Regulating Aspartate Aminotransferase in Myceliophthora thermophila During the Malic Acid-Aspartate Shuttle Pathway can Significantly Improve the Ability of Microorganisms to Synthesize Malic Acid

1. Construction of Aspartate Aminotransferase Expression Vector

The nucleic acid sequence C17941 (MYCTH_2314321) (SEQ ID NO.: 3) encoding aspartate aminotransferase was contained by PCR amplification using Myceliophthora thermophila genome as a template with primer pairs designed according to aspartate aminotransferase searched in the published genome database information (http://genome.jgi.doe.gov/Spoth2/Spoth2.home.html). After the nucleic acid sequence was digested with SpeI and EcoRI, it was ligated into linearized vector pAN52gpdA-C17941 which was digested with the same enzymes to obtain the recombinant plasmid and named as pAN52gpdA-C17941. The primers were shown as follows:

Cl7941-F: (SEQ ID NO.: 35) GGACTAGTATGGCGCCGACGTCAACAACG Cl7941-R: (SEQ ID NO.: 36) CGGAATTCTCATTGCACCTCCCGAACCAC

2. Determination of Malic Acid-Producing Capacity of Myceliophthora thermophila Transformant

The aspartate aminotransferase overexpression vector pAN52gpdA-C17941 was intergrated into Myceliophthora thermophila AS2 strain (which was intergrated with Myceliophthora thermophila transformant of Asmae overexpression vector derived from Aspergillus sojae, see example 2) genome, with a final concentration of 100 μg/mL and using G418 as the screening antibiotic. The method was shown in step 2 of Example 1.

All of the verified transformants were inoculated into a 250 mL conical flask containing 50 mL of medium with crystalline cellulose (Avicel) as the carbon source (see step 3 in example 1) at a inoculation amount of 2.5×10⁵/mL. The product was cultured at 45° C. in 150 rpm, and sampled on eighth day. After the sample was treated by the method as described in step 3.2 of example 1, the malic acid content in the fermentation broth was determined.

The results showed that the intergration of the aspartate aminotransferase into Myceliophthora thermophila AS2 can significantly promote the production of malic acid. The strain with the highest yield was named as CN201. On the eighth days the yield of malic acid was 69.2 g/L (FIG. 9), which was increased by 10% compared to the control strain AS2.

This example illustrated that the overexpression of the gene related to malic acid-aspartic acid shuttle pathway, i.e. aspartate aminotransferase can improve the ability of microorganisms to produce malic acid.

Example 7 Regulating Glutamic Acid-Aspartate Transporter Myceliophthora thermophila During the Malic Acid-Aspartate Shuttle Pathway can Significantly Improve the Ability of Microorganisms to Synthesize Malic Acid

1. Construction of Glutamic Acid-Aspartate Transporter Expression Vector

The nucleic acid sequence C11241(MYCTH_2300593)(SEQ ID NO.: 5) encoding aspartate aminotransferase was contained by PCR amplification using Myceliophthora thermophila genome as a template with primer pairs designed according to glutamic acid-aspartate transporter searched in the published genome database information (http://genome.jgi.doe.gov/Spoth2/Spoth2.home.html), and the primer pairs were:

Cl1241-F: (SEQ ID NO.: 35) GGACTAGTATGTCCAAGGCCGCAACTGTC Cl1241-R: (SEQ ID NO.: 36) CGGAATTCCTACGCCGTCTTTGCGTTCATC.

After the nucleic acid sequence was digested with SpeI and EcoRI, it was ligated into linearized vector pAN52-TN-MtPgpdA which was digested with the same enzymes to obtain the recombinant plasmid named as pAN52gpdA-C11241.

2. Determination of Malic Acid-Producing Capacity of Myceliophthora thermophila Transformant

The glutamic acid-aspartate transporter overexpression vector pAN52gpdA-C11241 was intergrated into Myceliophthora thermophila AS2 strain (which was intergrated with Myceliophthora thermophila transformant of Asmae overexpression vector from Aspergillus sojae, see example 2) genome, with a final concentration of 100 μg/mL and using G418 as the screening antibiotic. The method was shown in step 2 of Example 1.

All of the verified transformants were inoculated into a 250 mL conical flask containing 50 mL of medium with crystalline cellulose (Avicel) as the carbon source (see step 3 in example 1) at a inoculation amount of 2.5×10⁵/mL. The product was subjected to culture at 45° C. in 150 rpm, and sampled on eighth day.

After the sample was treated by the method as described in step 3.2 of example 1, the malic acid content in the fermentation broth was determined. The results showed that overexpression of Aspergillus sojae Asme and Myceliophthora thermophila glutamic acid-aspartate transporter (C11241) simultaneously can significantly promote the malic acid production in Myceliophthora thermophila. One of the strains was named as CN202, and on the eighth day the yield of malic acid was 66.9 g/L (FIG. 9), which was increased by 10% compared to the control strain AS2.

This example illustrated that the overexpression of gene glutamic acid-aspartate transporter related to malic acid-aspartic acid shuttle pathway can improve the ability of microorganisms to produce malic acid.

Example 8 Gene Deletion of Malic Acid-Alpha Ketoglutarate Transporter Gene could Improve the Ability of Myceliophthora thermophila Strain CN2 to Produce Malic Acid

(1) Amplification of Malic Acid-Alpha Ketoglutarate Transporter Gene and its Upstream and Downstream Homologous Arm Nucleic Acid Fragments

The upstream and downstream homologous arm nucleic acid sequences of the gene encoding malic acid-alpha ketoglutarate transporter UL and DL were obtained by PCR amplification using Myceliophthora thermophila genome as template with the primer pairs designed according to the sequence of malic acid-alpha ketoglutarate transporter gene (MYCTH_2081554, SEQ ID NO.: 91) and its upstream and downstream homologous arm nucleic acid searched in the published genome database information (http://genome.jgi.doe.gov/Spoth2/Spoth2.home.html), and the primer pairs were:

Cl4837-UF: (SEQ ID NO.: 39) GCTCTAGATGCTTGCAGGAACTCTCTGTGAAACC Cl4837-UR: (SEQ ID NO.: 40) GCGTTAACCCCACAGTTTGGAGAGACGACATCG Cl4837-DF: (SEQ ID NO.: 41) CCTTAATTAATGTATATACGGGGCGAATACGAAGG Cl4837-DR: (SEQ ID NO.: 42) CGGAATTCTTCCTCCTGCAAACTCAGCTTGAG. 

UL and DL were sequenced by Beijing Liuhe Huada Gene Technology Co., Ltd. and analyzed by NCBI Blast.

(2) Construction of a Vector with Malic Acid-Alpha Ketoglutarate Transporter Gene-Knocked Out

Sur gene fragment (GI:2547090) was amplified using plasmid pPK2surGFP as a template with the following primer pairs:

Sur-F: (SEQ ID NO.: 77) GCTCTAGAGTTAACGCGGCCGCGACTAGATCTGTGCCAACGCCACAG Sur-R: (SEQ ID NO.: 78) CGGAATTCGTTTAAACTTAATTAACCGACGGAATTGAGGATATCAGTCAC

PCR products and plasmid pPK2barGFP were digested with restriction endonuclease XbaI and EcoRI and then ligated by T4 DNA ligase to obtain plasmid pPK2sur-barGFP.

The above upstream and downstream homologous arm fragments of malic acid-alpha ketoglutarate transporter gene were obtained through PCR amplification and sequencing analysis. The PCR product of the upstream homologous arm was digested with restriction endonuclease XbaI and HpaI and the PCR product of the downstream homologous arm was digested with PacI and EcoRI. The plasmid pPK2sur-barGFP was digested with the same enzymes and then ligated with the upstream and downstream homologous arms using T4 DNA ligase to obtain the knock-out vector: pPK2sur-barGFP::odc (FIG. 7).

(3) The Vector pPK2sur-barGFP::Odc with Gene Knocked Out was Used to Transform Myceliophthora thermophila AS2 to Obtain Transformants

The transformant was verified by PCR with the primer CI4837-F2 located outside the upstream homologous arm at the 5′ end of gene of malic acid-alpha ketoglutarate transporter in Myceliophthora thermophila and Sur-R2 within Sur gene, the primers were as follows:

Cl4837-F2: (SEQ ID NO.: 87) CAGACTGTGTGGTTCTGCAACAGG Sur-R2: (SEQ ID NO.: 88) GGCCAACAGTACGAAGCATTTCG

PCR results showed that CI4837-F2 and Sur-R were able to amplify fragments of 3 KB size, which indicated that the Sur gene had been replaced malic acid-alpha ketoglutarate transportermalate encoding gene C4837.

At the same time, the transformant genome was amplified using ORF amplified primers CI4837-F and CI4837-R of malic acid-alpha ketoglutarate transporter encoding gene doc, and the sequences of the primers were as follows:

Cl4837-F: (SEQ ID NO.: 89) ATGGCGTCAGCAAAGGAGAAGG Cl4837-R: (SEQ ID NO.: 90) CTACGCCTCGCCATCCCTAATC

PCR result showed that no fragment was amplified by using primers CI4837-F and CI4837-R, which indicated that the obtained transformants were pure nuclei.

(4) Fermentation of the Transformants to Produce Malic Acid

250 mL triangle flask was used as a fermentation container, and the volume of the fermentation system in each was 50 mL of. The formulation of the fermentation medium of malic acid was as follows: microcrystalline cellulose 7.5%, peptone 6.0 g/L, 0.15 g/L KH₂PO₄, 0.15 g/L K₂HPO₄, 0.10 g/L CaCl₂.2H₂O, 0.10 g/L, MgSO₄.7H₂O, 80.0 g/L calcium carbonate, 1 ml/L trace elements solution (5 g NaCl, 5 g FeSO4.7H₂O, 1 g citric acid/L water).

32 transformants were collected by physiological saline solution. After filtered with 2-layer lens wiping paper, the number of spores was calculated, and the inoculation amount was 2.5×10⁵/ml. The transformants were cultured at 45° C. at 150 rpm and sampled on fourth, sixth and eighth days. After the samples were treated, the malic acid content was analyzed by HPLC. One strain was named as CN203, in which the yield of malic acid was 70.5 g/L (FIG. 9) on the eighth day. Compared to the control strain AS2, the yield of malic acid was increased by more than 10%.

Example 9 Overexpression of Glucose Transporter Gene in Myceliophthora thermophila can Increase Producing Capacity

1. Construction of Glt-1 Overexpression Vector (pAN52-Glt)

The glucose transporter encoding gene glt-1 (NCU01633, SEQ ID NO: 95) was amplified by PCR using cDNA under glucose condition from Neurospora crassa FGSC #2489 (purchased from Fungal Genetics Stock Center) as a template and primers. The primers were shown as follows:

GLT-F: (SEQ ID NO.: 93) CGGACTAGTATGGTCAAAGCTGTCGTTGCTG GLT-R: (SEQ ID NO.: 94) CGCGGATCCTCACTTCTGGGGGGGGTTGTG.

The gene was digested with SpeI and EcoRI and then ligated into the linearized plasmid pAN52-TB-MtPgpdA which was digested with the same enzymes, so as to obtain glt-1 expression vector, named as pAN52-glt.

2. Introducing the Expression Vector (pAN52-Glt) into Myceliophthora thermophila

The glt-1 overexpression vector pAN52-glt was linearized by BgI II and then intergrated into the Myceliophthora thermophila JG207 strains genome. The final concentration was 100 μg/mL. G418 was used as screening antibiotic. The method was shown in step 2 of example 1. The transformant was verified and obtained using primers MtPgpdA-F and GLT-R. The PCR system and methods were described in step 1.3 of example 1.

All of the verified transformants were inoculated into a 250 mL triangular flask containing 50 mL of medium with glucose and cellulose as the carbon source (see step 3 in example 1) at an inoculation amount of 2.5×10⁵/mL. The product was subjected to culture at 45° C. at 150 rpm. The sample was taken on the eighth day. After the sample was treated by the method as described in step 3.2 of example 1, the malic acid content in the fermentation broth was determined.

The results showed that the overexpression of glt-1 in malic acid-highly producing strain YG207 could significantly promote the capacity for producing malic acid and the strain with the strongest producing ability was named as JG207G. After four days of fermentation, the yield of malic acid was 42 g/L for glucose and 51 g/L for cellulose. Compared to the original strain JG207 (29 g/L), the yield was increased by 45% and 75%, respectively. The results showed that the overexpression of glucose transporter gene in Myceliophthora thermophila strain JG207 could effectively improve the capacity for producing malic acid by fermentation.

Example 10 Overexpression of C4-Dicarboxylic Acid Transporter in Neurospora crassa Failed to Obtain the Capacity for Producing Malic Acid at an Industrial Level

1. Construction of C4-Dicarboxylic Acid Transporter Gene Expression Vector

The sequence of nucleic acid encoding C4-dicarboxylic acid transporter gene Ncmae (NCU07517) (SEQ ID NO.: 13) was obtained by PCR amplification using Neurospora crassa genome as a template. The primers used for PCR amplification were NCU7517-F:GCTCTAGAATGGGCAGCCAGCCTCCCATGC (SEQ ID NO.: 79) and NCU7517-R:CCTTAATTAACTAATGATCCTCCACATCCTCA (SEQ ID NO.: 80). The sequence of nucleic acid encoding C4-dicarboxylic acid transporter gene mae (NCU07517) (SEQ ID NO.: 11) was obtained by PCR amplification using Aspergillus oryzae DSM1863 genome as a template. The primers used for PCR amplification were

Asmae-F: (SEQ ID NO.: 53) GCTCTAGAATGCTGACACCTCCCAAGTTTGAGGATG mae-2R: (SEQ ID NO.: 81) CCTTAATTAACTAATCAGATACATCCTCATCTTTACCC

The PCR product of C4-dicarboxylic acid transporter gene fragments obtained through PCR amplification and analysis and the plasmid pMF272 were digested by restriction endonuclease XbaI and PacI (the physical spectrum was shown in FIG. 8).

Then they were ligated by T4 DNA ligase to obtain the expression plasmid, named as pMF272-Nrmae and pMF272-mae respectively.

2. Integrating C4-Dicarboxylic Acid Transporter Encoding Gene into Neurospora crassa Genome

After expression vectors pMF272-Nrmae and pMF272-mae of C4-dicarboxylic acid transporter were transformed into Neurospora crassa FGSC9015, the verified transformants were all inoculated into 50 mL of medium in a 250 mL triangular flask containing D-glucose as the carbon source (formula: glucose 100 g/L, peptone 6.0 g/L, 0.15 g/L KH₂PO₄, 0.15 g/L K₂HPO₄, 0.10 g/L CaCl₂.2H₂O, 0.10 g/L MgSO₄.7H₂O, Calcium carbonate 80.0 g/L, and 1 ml/L trace elements solution (5 g NaCl, 5 g FeSO4.7H₂O, 1 g citric acid/L water) at an inoculation amount of 1×10⁶/mL, and cultured at 25° C. at 200 rpm. On the fourth day, the supernatant was tested to determine the malic acid content in the fermentation broth. The results showed that when Ncmae from Neurospora crassa was expressed in Neurospora crassa FGSC9015, the highest yield of malic acid was 2.7 g/L and when mae from Aspergillus oryzae was expressed in Neurospora crassa FGSC9015, the highest yield of malic acid was 2.5 g/L. Compared to the control strain Neurospora crassa FGSC9015 (with a yield of 1.5 g/L), the expression of C4-dicarboxylic acid transporter was increased, but it could not meet the needs of industrial applications.

This experiment showed that the capacity for producing malic acid can not be effectively increased to an industrial level by modifying the malic acid synthesis pathway (such as overexpression of malate transporter) in these non-dominant strains of organic acids accumulation, such as Neurospora crassa. Although it had been reported that in Aspergillus the dominant strain of organic acid accumulation, the capacity for producing malic acid can be effectively increased to an industrial level by modifying the malic acid synthesis pathway, the capacity could not be promoted to these non-dominant strains of organic acid accumulation.

Example 11 Overexpression of C4-Dicarboxylic Acid Transporter in Trichoderma Reesei Failed to Improve the Ability of Microorganisms to Produce Malic Acid

1. Construction of C4-Dicarboxylic Acid Transporter (SEQ ID NO.: 11) Overexpression Vector in Aspergillus oryzae

Encoding reading frame mae (SEQ ID NO.: 11) of C4-dicarboxylic acid transporter gene was amplified by PCR from cDNA of Aspergillus DSM1863, and the primers were as follows:

Amae-F: (SEQ ID NO.: 82) TTCCAACTAGTATGCTGACACCTCCCAAG Amae-R: (SEQ ID NO.: 83) AATGGTTAACCTAATCAGATACATCCTC

After the PCR reaction, the PCR product was digested with restriction endonuclease SpeI and HpaI and ligated into the SpeI and HpaI digestion sites of plasmid pCY01 (containing hygromycin resistance gene, and on both sides of the polyclonal digestion site are promoters of elongation factor of Myceliophthora thermophila and terminators of Aspergillus trpC) to obtain plasmid pNEO-Amae. Plasmid pNEO-Amae was transformed into Trichoderma reesei QM6a by protoplast method, and the obtained transformants were QM6a-Amae.

2. Detecting the Acid-Producing Ability of the Recombinant Strains Obtained by Overexpressing C4-Dicarboxylic Acid Transporter in Trichoderma reesei

1.25×10⁷ spores were inoculated to 50 mL of acid-producing medium which was the same as that described in Example, and the product was cultured in 150 rpm at 28 degrees for 8 days. 1 mL of fermentation liquid was added to the 1 mL of 2M sulfuric acid, and reacted at 80 degrees for 20 min. To the above was added 2 mL of water, and centrifuged at 14000 rpm for 10 min after mixing. The supernant was tested to determine the content of malic acid using HPLC as described in Example 1. The yield of malic acid in the original strain was 2.5±0.6 g/L and that in the transformant was 2.4±0.4 g/L.

The results showed that the expression of C4-dicarboxylic acid transporter in Trichoderma reesei failed to improve the ability for producing malic acid to industrial level.

This experiment showed that the capacity for producing malic acid can not be effectively increased to an industrial level by modifying the malic acid synthesis pathway (such as overexpression of malate transporter) in these non-dominant strains of organic acids accumulation, such as Trichoderma reesei. Although it had been reported that in Aspergillus the dominant strain of organic acid accumulation the capacity for malic acid can not be effectively increased to an industrial level by modifying the malic acid synthesis pathway, the capacity could not be promoted to these non-dominant strains of organic acid accumulation.

Example 12 Overexpression of Mae and Pyc in Thermophilic Fungi Myceliophthora heterothallica Enables the Strain have the Ability to Produce Malic Acid

This example showed that when pyruvate carboxylase and C4-dicarboxylic acid transporter were expressed in M. heterothallica, the obtained recombinant microorganisms could significantly increase malic acid-producing capacity.

The vector pAN52-mar-pyc expressing mae and pyc (the construction of which was described in Step 1 of Example 1) was screened to obtain several positive transformants in the protoplast transformed strain M. heterothallica CBS202.75 with hygromycin gene hph as the selective marker. The positive transformants were subjected to malic acid fermentation with 7.5% microcrystalline cellulose Avicel as the substrate and the medium composition was found in Example 1, Step 3. With M. heterothallica CBS202.75 as a reference, the yield of malic acid on the eighth day of the fermentation had been increased up to 47.4 g/L.

This experiment showed that the modification through metabolic engineering could significantly improve the ability to synthesize malic acid in Trichoderma harzianum. Compared with Examples 9 and 10 of the present invention, it could be found that the modified Myceliophthora strains could significantly improve the organic acid (malic acid) synthesis, however, which was not predictable.

Example 13 Establishment of Fermentation Process to Produce Malic Acid in Recombinant Myceliophthora thermophila

1. Culturing the spore: The recombinant Myceliophthora thermophila JG207 was fermented in 5 L fermentor (BIOTECH-5JG, Shanghai Baoxing Biological Equipment Engineering Co., Ltd.) as follows: recombinant Myceliophthora thermophila JG207 was inoculated into MM plate medium and the plates were placed in an incubator at 45° C. for 8 days, and spores were washed with 0.8% NaCl and 0.1% Tween-80 and counted.

2. Culturing the seed liquid: 2.5×10⁷ spores were transferred to a 250 mL triangle flask containing 100 mL of seed medium, which was cultured at 45° C. in 150 rpm for 24 h and then the obtained liquid was taken as the seed for fermentation. The seeds were fermented using a synthetic medium. In a 5 L fermentation tank was loaded with 3.3 L fermentation medium and 400 mL seed liquid.

The composition of MM solid medium (per liter) was 20 g sucrose, 20 mL 50× Vogel's salt, and 15 g agar. The composition of 50× Vogel's salt (g/L) was: 125 g Na₃citrate. 2H₂O, 250 g KH₂PO₄, 100 g NH₄NO₃, 10 g MgSO₄.7H₂O, 0.1 g CaCl₂. 2H₂O, 5 mL trace element solution, 2.5 mL Biotin, and 755 mL water.

The composition of seed medium (per liter) was 10 g glucose, 0.15 g K₂HPO₄, 0.15 g KH₂PO₄, 0.1 g MgSO₄.7H₂O, 0.1 g CaC12, 6 g Bacto, peptone, 1 mL trace element solution. The composition of trace element solutions (g/L) was: 5 g, Citric acid. 1H₂O, 5 g ZnSO₄.7H₂O, 1 g Fe(NH₄)₂(SO₄)₂.6H₂O, 0.25 g CuSO₄.5H₂O, 0.05 g MnSO₄.1H₂O, 0.05 g, H₃BO₃, and 0.05 g Na₂MoO₄.2H₂O.

The composition of fermentation medium (per liter) was: 75 g carbon source, 80 g CaCO₃, 0.15 g K₂HPO₄, 0.15 g KH₂PO₄, 0.1 g MgSO₄.7H₂O, 0.1 g CaCl₂, 6 g Bacto peptone, 0.5 mL Biotin, and 1 mL trace element solution.

The composition of feeding medium (per liter) was 0.45 g K₂HPO₄, 0.45 g KH₂PO₄, 0.3 g MgSO₄.7H₂O, 0.3 g CaCl₂, 18 g Bacto peptone, 1.5 mL Biotin, and 3 mL trace element solutions.

3. Fermentation process: fermentation temperature 45° C., air flow rate 4 L/min, and the dissolved oxygen was control at 30%. In order to control the dissolved oxygen at 30%, the speed needed to be coupled with the dissolved oxygen, and kept at 200-800 rpm. During the fermentation process calcium carbonate was added to control pH at more than 6.

In the 48th hour during fermentation the feeding medium was fed through simulated exponential feeding method with an average feeding rate of 8 mL/h. In the 72 h, 96 h, 120 h, 144 h, 168 h, 192 h, 216 h, and 240 h during the fermentation, 60 g of carbon source was supplemented respectively.

After being fermented for 48 h, every 24 h 1 mL of bacteria liquid was taken and 1 mL of 2M H₂SO₄ was added, the mixture was mixed and then treated at high temperature of 80 for 25 min, then to the mixture was add 1 mL of sterile water. The mixture was centrifuged in 14000 rpm for 10 min. The supernatant was tested to determine the content of malic acid using HPLC (Waters e2695 HPLC).

The fermentation cycle is 240 h-264 h and the yield of malic acid can be increased continuously.

The method for producing malic acid by fermentation in recombinant Myceliophthora thermophila strains can be conducted with a variety of carbon sources as substrate and was consistent with the above methods. The yield of malic acid was 230 g/L with glucose as the carbon source. The yield of malic acid was 168 g/L with Avicel as the carbon source. The yield of malic acid was 95 g/L with corn stalk as the carbon source.

Example 14 Separation and Preparation of Malic Acid

The separation and preparation of malic acid was generally divided into three steps: extraction of crude malic acid, refinement, and crystallization.

1. Extraction of crude malic acid: the fermentation broth was processed by six steps such as acid hydrolysis, filtration, neutralization, filtration, acid hydrolysis and filtration, so as to obtain the crude malic acid solution. The fermentation broth was put in the acid hydrolysis tank, and then adjusted to pH1.6 using sulfuric acid, and the acid hydrolysis should be carried out with stirring. After the acid hydrolysis was completed, the gypsum slag, bacteria and other precipitates were filtered by plate-and-frame filter press. The filtrate was put in the neutralization tank, and adjusted to pH 7.5 by adding CaCO₃ solid and lime milk. The neutralization liquid was placed in the settling tank for 7 h to allow the calcium malate in the solution crystallize sufficiently. After the above calcium salt system was clear, the supernatant was removed. Then theremain was filtered by releasing the filter tank bottom, and the cake was washed with a small amount of cold water to remove most of the soluble impurities. The calcium malate was transferred into the acid hydrolysis tank, to which was added 2 times the weight of warm water. The mixture was stirred into a suspension, adjusted to pH 1.6 by adding sulfuric acid, stirred sequentially for about half an hour, and then stood for several hours, so that the precipitation of gypsum slag was fully precipitated. The gypsum slag in the above system was filtered by the pressure filter. The filtrate was crude malic acid solution containing trace succinic acid and other organic acids, as well as Ca²⁺, Mg²⁺ and other metal ions and pigments, and would be refined in the next step.

2. The refinement of malic acid: ion exchange and activated carbon were combined used for the treatment. The mother liquid of malic acid was successively purified by 5-column purification system comprising CAL granular activated carbon decolorization column, cation exchange resin 732, anion exchange resin D315, BPL column activated carbon adsorption column and cation exchange resin. During the processing, the crude malic acid solution passed through the 5 column system in sequence, and the liquid flew top-down with a flow rate of 7 to 8 L/min. The effluent was tested to monitor the unsaturated fatty acid content by an ultraviolet absorption analyzer. If there was unsaturated fatty acid in the effluent, it should be reprocessed to the anion exchange resin D315 column. The CAL granular activated carbon decolorization column was used for decoloration and removing some unsaturated fatty acids. Cation exchange resin 732 was used to remove metal ions. Anion exchange resin D315 was used to remove succinic acid and other anions.

The malic acid solution with high purity was reduced and concentrated at 70° C. Then it was cooled down to 20° C., and some crystal seeds were added and crystallization was conducted with slow stirring. After 3 h, malic acid was crystallized.

The malic acid crystals was dried under vacuum at a temperature controlled between 40˜50° C.

Example 15 the Transformation of Wild Strains with Organic Acid Accumulation Ability and the Detection of their Capacity for Producing Organic Acid

In this study, vectors overexpressing malate dehydrogenase, aspartate aminotransferase and glutamic acid-aspartate transporter were constructed respectively and were used to transfer Aspergillus (including Aspergillus niger, Aspergillus sojae, Aspergillus oryzae) possessing capacity for accumulating organic acid to obtain multiple transformants and glucose was used as the reaction substrate. The method was shown as the method for the combination of construction of each transformant described in the above examples. Their products and yields were identified. Among them, the numbers of the engineered strains upon transformation and the products were shown in table 3:

TABLE 3 Name of the original engineered strain Characteristics of the engineered strain strain PM101 overexpression of malate dehydrogenase Aspergillus oryzae PM102 Overexpression of malate Aspergillus dehydrogenase + aspartate aminotransferase oryzae PJ103 overexpression of malate dehydrogenase aspergillus sojae TJ104 Overexpression of aspartate aspergillus aminotransferase sojae GJ105 Overexpression of glutamic acid-aspartate aspergillus transporter sojae GM106 Overexpression of glutamic acid-aspartate Aspergillus transporter oryzae

The identification results showed that after the corresponding genes were transformed, the capacity for producing malic acid in Aspergillus sojae and Aspergillus oryzae had been significantly improved, reaching more than 20-60 g/L respectively, wherein the PM102 strain with two genes transformed provided better performance.

Therefore, the gene modification of the invention could significantly improve the acid-producing capacity of the strain which had an accumulation effect for dibasic acid in its original strain.

Discussion

For the fermentation of organic acids such as malic acid, the traditional dominant strains are Aspergillus strains (preferably Aspergillus niger-citric acid, itaconic acid-Aspergillus terreus, malic acid-Aspergillus flavus, Aspergillus oryzae) and Rhizopus strains (Rhizopus oryzae-lactic acid), while Trichoderma and Neurospora strains do not belong to the strains that commonly accumulating organic acid. The test has showed that for these strains without significant accumulation of organic acids under the natural conditions, it usually can not effectively improve the production to an industrial level (10 g/L or more) by modifying their organic acid (such as malic acid) synthetic pathway. It has showed that this conclusion can not be applied to the entire filamentous fungi although the modification of synthetic pathway of organic acids in Aspergillus has significantly improved the synthesis of organic acids, especially for those non-organic acid accumulation strains, including Myceliophthora. it can not be speculated if they have the capacity to synthesize the organic acid at an industrial level. More specific experimental researches are needed. It was unexpected that the inventors of the present invention had firstly demonstrated that Myceliophthora strain did not accumulate organic acids in a large amount (usually no more than grams/liter) in the culture medium under natural conditions but can provide industrialized malic acid fermentation ability (10-100 g/I and above) after transformation. Therefore, the present invention has great contingency and innovativeness.

Meanwhile, the inventors have found that not only the malic acid (and even organic acid) fermentation in Myceliophthora can be enhanced through regulating multiple new genes, but also the organic acid-producing capacity of strains including Aspergillus (preferably Aspergillus oryzae, Aspergillus sojae) and so on (beside Myceliophthora with accumulation ability of the organic acid) can be improved by genetic modification.

In addition, the inventors have established and optimized the high temperature fermentation process of Myceliophthora thermophila, including the fermentation feeding process by using solid biomass such as glucose and other soluble sugar fermentation process and cellulose and the like as the carbon source.

All documents mentioned in the present invention are incorporated herein by reference, as if each document were individually recited for reference. It should be understood that those skilled in the art will be able to make various changes or modifications to the present invention after reading the teachings of the present invention, which also fall within the scope of the claims appended hereto. 

The invention claimed is:
 1. An engineered strain with genetic modification for synthesizing a dibasic organic acid, comprising: an expression product or upregulated expression product of a positive regulator gene for synthesizing a dibasic organic acid, and/or a down-regulated expression product of a negative regulator gene for synthesizing a dibasic organic acid, wherein the engineered strain has an improved capability for producing the dibasic organic acid as compared to an original strain of the engineered strain, the dibasic organic acid is selected from the group consisting of malic acid, succinic acid, fumaric acid, oxaloacetic acid, glutaric acid, adipic acid, and combinations thereof; the expression product or upregulated expression product of the positive regulator gene comprises one or more polypeptides selected from the group consisting of aspartate aminotransferase, glutamate-aspartate transporter, and derived polypeptides thereof, or the expression product or the upregulated expression product of the positive regulator gene is C4-dicarboxylic acid transporter, wherein the aspartate aminotransferase has the amino acid sequence of SEQ ID NO: 4, the derived polypeptide of the aspartate aminotransferase exhibits an aspartate aminotransferase activity and has a sequence identity of ≥90% to the SEQ ID NO: 4, the glutamate aspartate transporter has the amino acid sequence of SEQ ID NO: 6, the derived polypeptide of the glutamate aspartate transporter exhibits a glutamate aspartate transporter activity and has a sequence identity of ≥90% to SEQ ID NO: 6 and the C4-dicarboxylic acid transporter has the amino acid sequence selected from the group consisting of SEQ ID NO: 14, 16, 18, 22, and combinations thereof; the down-regulated expression product of the negative regulatory gene includes one or more polypeptides selected from the group consisting of Succinyl-CoA synthase, Malic acid-alpha ketoglutarate transporter, and derived polypeptides thereof, wherein the Succinyl-CoA synthase has the amino acid sequence of SEQ ID NO: 2, the derived polypeptide of the Succinyl-CoA synthase exhibits a Succinyl-CoA synthase activity and a sequence identity of ≥90% to SEQ ID NO: 2, the Malic acid-alpha ketoglutarate transporter has the amino acid sequence of SEQ ID NO: 8, and the derived polypeptide of the Malic acid-alpha ketoglutarate transporter exhibits a Malic acid-alpha ketoglutarate transporter activity and a sequence identity of ≥90% to SEQ ID NO: 8; and the engineered strain comprises Myceliophthora.
 2. The engineered strain of claim 1, wherein the engineered strain comprises Myceliophthora thermophila, or Myceliophthora heterothallica.
 3. The engineered strain of claim 1, wherein, as compared to the original strain, the engineered strain provides a fermentation yield of dibasic organic acid at more than 10 g/L, based on a volume of fermentation liquid, and/or increases or improves the dibasic organic acid producing capacity by 10-50%.
 4. The engineered strain of claim 1, wherein the expression product or upregulated expression product of the positive regulator gene further comprises one or more peptides selected from the group consisting of pyruvate carboxylase, malate dehydrogenase, glucose transporter, and derived polypeptides thereof, and the derived polypeptides thereof have a sequence identity of ≥90% and exhibit similar activity to the pyruvate carboxylase, malate dehydrogenase, or glucose transporter.
 5. The engineered strain of claim 4, wherein the expression product or upregulated expression product of the positive regulator gene further comprises: the pyruvate carboxylase of SEQ ID No: 26; the malate dehydrogenase of SEQ ID NO: 10; and/or the glucose transporters of SEQ ID NO:
 96. 6. A method for preparing a dibasic organic acid comprising steps of (i) providing the engineered strain of claim 1; (ii) in presence of a substrate, culturing the engineered strain described in (i), thereby obtaining a fermentation product containing a dibasic organic acid; and optionally (iii) isolating and purifying fermentation product obtained in (ii), thereby obtaining the dibasic organic acid.
 7. The method of claim 6, wherein the substrate comprises monosaccharides, polysaccharides, glycans, biomass, or combinations thereof.
 8. The method of claim 6, wherein in the step (ii), the culture temperature is 40-55° C.
 9. The method of claim 6, wherein in the step (ii), the culture temperature is 45-50° C.
 10. The method of claim 6, wherein in the step (ii), the culture temperature is 25-60° C.
 11. The engineered strain of claim 1, wherein the expression product or upregulated expression of the positive regulator gene is selected from the group consisting of the aspartate aminotransferase of SEQ ID NO: 4, and the glutamate aspartate transporter of SEQ ID NO:
 6. 12. The engineered strain of claim 1, wherein the down-regulated expression product of the negative regulator gene is selected from the group consisting of the succinyl-CoA synthase of SEQ ID NO: 2, and the malic acid-alpha ketoglutarate transporter of SEQ ID NO:
 8. 13. The engineered strain of claim 1, wherein, as compared to the original strain, the engineered strain provides a fermentation yield of dibasic organic acid at 10-50 g/L, based on a volume of fermentation liquid.
 14. The engineered strain of claim 1, wherein, as compared to the original strain, the engineered strain provides a fermentation yield of dibasic organic acid at 50-300 g/L, based on a volume of fermentation liquid.
 15. The engineered strain of claim 1, wherein, as compared to the original strain, the engineered strain increases or improves the dibasic organic acid producing capacity by 50%-500%. 